Overall, this set is a cubelet builder's dream. There are so many cards with tons of unique play that almost any card from the set is viable for a cubelet. If you are new to the format or getting tired of figuring out which cuts to make to include new cards, this set has enough to serve as a core for your cubelet. The list provided in this post are those that are just a cut above the rest and would be the ones to seriously include if you are updating a "Legacy" level cubelet.

Esper Sentinel
Annoying to use a removal spell for can gain you some much needed card advantage.

Out of Time
Has the potential to create an exceptionally chaotic board, which is always an exciting element to cubelet.

Prismatic Ending
Fairly safe removal spell that will get most permanents in a cubelet, and the cost is fair as it scales.

Subtlety
More than likely, the creature/planeswalker will end up on the bottom of the library, which is interesting for blue in a cubelet.

Bone Shards
Planeswalkers are always difficult to put into a cubelet, but this gives one more answer to make them viable.

Damn
Being able to be a spot removal or wrath may open up one card slot in the 100 with this flexible option.

Ragavan, Nimble Pilferer
It's monke, what more can I say that hasn't already been said.

Endurance
A unique way to manage the shared graveyard.

Jade Avenger
Bushido is an interesting mechanic that doesn't see a whole lot of play, but this "bear" could see some good play.

Dakkon, Shadow Slayer
Surveil is powerful, the minus is fair and the ultimate isn't overpowering if you built your cubelet correctly. Great planeswalker to include if that is your thing.

Geyadrone Dihada
Similar power and potential as Dakkon. Demands attention from the field. Fair inclusion.

Master of Death
Interesting interactions with the field and surveil is a strong mechanic.

Lessons and Learn - Lessons can be treated one of two ways with regards to Learn. The easy way is just like Commander, there is no sideboard or wishboard, so you would not be able to search for a lesson when you learn.  The other way would be to have a shared binder that if you incorporate Learn into your cubelet, everyone has access to the lessons and it is a first come, first serve on the lessons. Lessons listed below are assessed as if they were just normal spells and not specifically tied to the Learn mechanic.

Guiding Voice
Simple way to learn.

Leonin Lightscribe
In a combat centric cubelet, this seems like a great inclusion.

Show of Confidence
Interesting spell if you can get your storm count up.

Study Break
Another nice way to learn while being a neat way to mess with combat.

Curate
Not as powerful as Ponder, but a nice substitute if you are looking for one.

Multiple Choice
Sweet options and powerful if you get to do all of them.

Confront the Past
Two strong options, especially if you have planeswalkers in your cubelet.

Eyetwich
Simple little flying creature that gets you a little value when it dies. Kind of like a delayed Baleful Strix.

Draconic Intervention
A unique board wipe that will clean up problematic creatures by exiling them if they die.

Extus, Oriq Overlord / Awaken the Blood Avatar
Both sides provide two interesting strategies. Both are powerful, but can be contained.

Plargg, Dean of Chaos / Augusta, Dean of Order
Both sides provide two interesting strategies. Both are powerful, but can be contained.

Shaile, Dean of Radiance / Embrose, Dean of Shadow
Having to choose between these two gives some neat flexibility to the game play. Embrose feels strong with a +1/+1 counter strategy.

Deadly Brew
With a shared graveyard, you have access to everything that was sacrificed.

Expressive Iteration
An unique twist on card selection. Because you can "play" the cards, you can make the one you exiled a land if you haven't played a land that turn.

Fracture
A great inclusion if you are light on artifact, enchantment, and/or planeswalker removal

Lorehold Command
Great modes. Lots of flexibility.

Prismari Command
Great modes. Lots of flexibility.

Quandrix Command
Great modes. Lots of flexibility.

Silverquill Command
Great modes. Lots of flexibility.

Tend the Pests
Get some mileage out of your creatures by suddenly making more

Witherbloom Command
You guessed it. Great modes (except the first one). Lots of flexibility.

Foretell - Similar to adventures in Throne of Eldraine, you would have your own individual zone for the cards you foretell, should you decide to add this mechanic to your cubelet.

Rune Cycle - Each seem fine, not too overwhelming for a cubelet. Could be a nice addition to an equipment/auras matter cubelet.

Snow - Snow is a house rule, but generally speaking, the lands produced from placing a card face down do not make them snow lands. Obviously, if you are building a snow matters cubelet, that rule can and should be changed. Cards listed in the set snapshot will not include cards like Ascendant Spirit because snow is not part of the basic cubelet.

Clarion Spirit
Cubelet rewards the ability to cast multiple spells in a turn later in the game, so this might be a nice way to reward and look to close out a game.

Divine Gambit
Exceptional answer to a lot of problems and adds a little chaos and entertainment factor. Punishing if opponent has no permanents in hand.

​Doomskar
Excellent boardwipe spell that can upgrade one of the other boardwipes, especially if adding additional foretell cards.

Halvar, God of Battle
If you have an equipement/auras matter cubelet, seems like a perfect fit, otherwise just a generically powerful card on either side.

Alrund, God of the Cosmos
Just the right level of entertainment without being too overly powerful.

Cosima, God of the Voyage
With a shared exile, this card would get passed around if people want to play that subgame. The Omenkeel won't exile lands, so you won't be playing anything with it in play.

​Egon, God of Death
With a shared graveyard, it could be well supplied to keep Egon around and the back side isn't overly powerful.

Valki, God of Lies
Relatively fair for the cubelet. Tibalt might be too powerful, but worth the test.

Tibalt's Trickery
Unique counterspell and with an entire deck full on nonlands, you are going to get your value.

Esika's Chariot
Fine vehicle to add to any cubelet that has a respect for artifacts and can answer it.

Sarulf's Packmate
Limited all-star that looks great in a Pauper cubelet.

Toski, Bearer of Secrets
Very annoying, but if you have exile effects, would be fun to add to the game.

Frija's Retribution
Decent way to close out a game or turn the game around.

Sarulf, Realm Eater
Pernicious Deed might be too powerful, but the conditions on Sarulf make it seem more manageable.

Commander Legends brings a lot of potential for Cubelet. Cubelet is largely a multiplayer format. When looking at what makes a multiplayer format good, there is a lot of crossover between it and Commander. The two mechanics that seem to be standouts are Encore and Monarch. When designing your cubelet, these two could be dramatic focuses/themes of the cubelet or could be a splash mechanic in your overall construction.

One thing to note: there are a group of cards that reference if you control your commander, which definitely does not apply to this format, so they would be evaluated as if you didn't control your commander.

Vow Cycle
Interesting cycle of cards that can be political tools. Generally speaking, they are fine and you will probably want to have access to all of them or none of them.

Court of Grace
When you are not the monarch, this card seems fine. When you are the monarch, this card seems absurd without some sort of way to balance it.

Court of Cunning
Not ideal for this format due to shared library.

Fall from Favor
Powerful effect and an easy way to introduce Monarch into your game.

Court of Ambition
Might be too powerful, but worth a test.

Exquisite Huntmaster
The first time, it is basically a different kind of Flametongue Kavu, but gets interesting when you are able to cast it for Encore.

Court of Ire
Very powerful. Essentially a removal spell or a quick way to eliminate a player if you are the monarch.

Krark, the Thumbless
Adds a level of chaos to the game as long as it sticks around.

Toggo, Goblin Weaponsmith
Because you control your land drops, you control how important rocks are to you,

Wheel of Misfortune
Interesting mini-game to play with the group. Might be fun to have in the deck for the first few plays, then look to swap it out or always make it a land.

Court of Bounty
Fundamentally does not work as your hand won't have lands in your hand when you aren't the monarch. This could mean that the card is fine since you can really only do something when you are the monarch.

Halana, Kessig Ranger
Having the ability to pay to have your creatures bite other creatures is unique and could be fun.

Araumi of the Dead Tide
Worth testing, might be too powerful. Just the kind of card you look for in Cubelet.

Belbe, Corrupted Observer
Everyone is hungry for mana, so this might actually stick around for a turn or two.

Gor Muldrak, Amphinologist
Just the right level of crazy/interesting for Cubelet

Modal Double-Faced Cards (DFC): Cubelet is no stranger to this idea as basically every card in the cubelet flips over to a land that can be any color of mana. Because of this, if a DFC card is mentioned in this set review, it based solely on the spell side of the card.

Party: A neat mechanic and could easily be the foundation for a cubelet. So many great cards available from the history of Magic and this set creates payoffs if you choose to build your cubelet to party.

Canyon Jerboa
You will only get one landfall activation a turn, so it isn't overpowered. It rewards consistent land drops and can help close a game.

​Luminarch Aspirant
Being able to help get a creature out of Slagstorm range or other damage based can be powerful and if this card lives for a few turns, you get some great value.

Maul of the Skyclaves
Very powerful, but if your cubelet is tuned to be able to remove artifacts from the board, you could look at making this an exceptional inclusion.

Nahiri's Binding
Interesting option as you can attach it to a planeswalker to render them useless.

Inscription of Insight
Any one of the options is fair for the four mana it takes to cast. If you have the additional four mana to kick this spell, then you should be rightfully rewarded.

Jace, Mirror Mage
This is a great addition to a cubelet. Scry is a very powerful in cubelet, and you will likely become the target until Jace is removed, but you do not win the game outright because of Jace. 

Feed the Swarm
Target removal for enchantment is something to consider for a pauper or peasant cubelet.

Inscription of Ruin
Mind Rot effects are a little feel bad, but each of the options are fair and being able to do all of them when this inscription is kicked seems pretty awesome.

Mind Carver
Dangerous with the shared graveyard, but if the cubelet is designed to handle powerful artifacts, then this seems fun.

Nighthawk Scavenger
Vampire Nighthawk is already a very playable card in cubelet, so the question you need to ask is, "Do I want to go bigger?"

Scourge of the Skyclaves
Casting this with kicker is going to be an absolute beating, but with no evasion, this card could be an interesting option.

Skyclave Shade
Another one of those, "this is funny because we share the graveyard" kind of options. You don't need to many of those in your cubelet, but it is an option.

Taborax, Hope's Demise
Being just a 2/2 is not an immediate threat, however, things can get out of hand if not watched.

Kargan Intimidator
A very interesting card with different options. Worth trying out in a cubelet.

Relic Robber
Could create for a very silly politics.

Valukut Exploration
A particularly powerful enchantment that if have not designed your cubelet to handle, this will give the person who owns this a tremendous amount of advantage, even if only able to trigger landfall at most once per turn.

Wayward Beast
If you play with the belief that the cards you decide to be lands are lands for the rest of the game, this isn't worth it. If you play a little more of a broken game where you are able to manipulate your lands, then this is an exceptional addition.

Broken Wings
An upgrade or flexible option for a pauper or peasant cube.

Inscription of Abundance
Just like the other inscriptions, each of these options are fine and a nice reward if you can cast it with kicker.

Spare Supplies
A neat card to help add to delirium while being only an okay artifact.

Basri Ket
Planeswalkers are very powerful, and their inclusion into a cubelet needs to take very seriously in order to promote healthy game play. That said, this is a pretty safe card to consider. To make the most of this walker, you will need to be on the aggressive side, which it then becomes difficult to defend against multiple players. The emblem, should you obtain it, will end the game, but if you played your cards right, you should be in a "win more" situation.

Basri's Lieutenant
Protection from multicolor is strong in a cubelet. Having some resilience to board wipes is also a bonus. Are the two together too strong when you have cards than can exile this?

Jeskai Elder
If Looter Il-Kor is too strong or annoying, might I suggest a Granny with prowess?

Sublime Epiphany
Too many options, but at six mana, is that a bad thing?

Teferi, Master of Time
Let's talk about Teferi. Any player's turn is bonkers in multiplayer and very strong in 1v1. You have to have very well balance cubelet to support this power, or you have to be doing lots of broken things where this card seems normal.

Demonic Embrace
Having a shared graveyard makes this card interesting and having to pay three life will help progress the game. Passing this around is not the end of the world if that is a subgame that wants to be played.

Eliminate
There are better removal spells in a Legacy cubelet, but this is a fine inclusion for a Peasant cubelet.

Hooded Blightfang
There are a few deathtouch creatures, so getting extra triggers is possible. The important part is putting pressure on planeswalkers if you are putting them into your cubelet.

Liliana, Waker of the the Dead
Similar to Teferi, Liliana in multiplayer is very powerful and her ultimate with a shared graveyard is just incredible. Take caution when trying to include this card.

Liliana's Standard Bearer
A "got you" card against the board wipes in the cublet. It's a fair card. A good card? Maybe.

Village Rites
A one-time skull clamp is hilariously fair in whatever cubelet you decide to build.

Garruk, Unleashed
Similar to Basri, this is an okay card. If your playgroup has the "no tutor" rule, then this card fails at it's ultimate.

Garruk's Harbinger
This is a strong creature card while having several options for removal.

Jolrael, Mwonvuli Recluse
Drawing additional cards is a staple of the cubelet, but I don't see Jolrael running away with a game. Her ability is strong but it is so easy to remove her before it becomes an issue.

Sporeweb Weaver
Slows down the aggro game, but it having reach can be very relevant an appealing for the inclusion.

Mutate Creatures
As a whole, the mutate mechanic is an extremely interesting mechanic and an entire cubelet could be designed around mutate containing one copy of all the mutate creatures and additional spells to round out the experience. Any creature listed here would likely be the only mutate creature in the deck, so its effect is only going to normally happen once in a game.

Luminous Broodmoth
A powerful mythic that adds resiliency to one player. Will likely make them the target until Broodmoth is removed from the game.

Mythos of Snapdax
A unique boardwipe that puts all the power into the caster's control as you will always have the ability to choose.

Mythos of Illuna
Copy spells that fight could create a unique experience.

Sea-Dasher Octopus
If cards like Looter Il-Kor are annoying, but you want a similar effect, this would be a good substitute.

Extinction Event
A unique boardwipe that also exiles, which can be essential.

Heartless Act
A removal spell with options.

Mythos of Nethroi
Three-mana remove any nonland permanent is very fair and acceptable in Cubelet.

Mythos of Vadrok
Neat way to force through damage when the board stalls. It lasts until your next turn, which adds extra politics to the game.

​Gemrazer
Artifacts and enchantments can be very annoying and this could be great to keep them in check.

Mythos of Brokkos
Usually tutors are frowned upon, so this mythos is not necessarily approved for cubelet, but you are welcome to try it.

Boneyard Lurker
A gravedigger grabbing a permanent is a fair card.

Brokkos, Apex of Forever
Shared graveyard makes this card very interesting if it ever ends up in the graveyard.

Chevill, Bane of Monsters
Not powerful, but can add a fun subgame for the player that casts Chevill.

Lore Drakkis
If you are looking for a Snapcaster like effect, this could be a great solution. It is not the same as Snapcaster Mage, but it works.

Skycat Sovereign
Great mana sink for the late game when you are trying to get those last few points of damage.

Crystalline Giant
Adds a unique subgame and adds an artifact to the deck for those that play delirium.

The Theros Gods
A feature/highlight of the set, the Theros Gods are extremely powerful in Cubelet and either need to be designed around to balance them or ignore them.

Omens
A cycle unique to Theros Beyond Death, as a whole, the Omens, besides Omen of The Hunt are decent enough to consider for a pauper cubelet. Omen of the Hunt does not work with the format.

Alseid of Life's Bounty
The one-time protection can get be very timely and being able to add enchantment to any delirium strategies is nice as well.

Elspeth Conquers Death
Potentially too strong, but there is a lot of utility and for the price, it might balance it out.

Elspeth, Sun's Nemesis
Not a particularly strong planeswalker, but the escape mechanic can help clean up the graveyard of any annoying cards.

Taranika, Akroan Veteran
Rewards playing creatures early to apply pressure on the opponent's life total.

Nadir Kraken
There's not a tremendous amount of draw spells, but if ignored, can create issues for opponents.

Thassa's Intervention
A wicked late game spell, but the flexibility of being a counterspell or a card selection spell is something to consider.

Cling to Dust
Helps control the graveyard while rewarding you with life or cards. An interesting substitute if you aren't running a card like Scrapheap Scrounger.

Pharika's Libation
If designing an enchantment heavy cubelet, it having two modes is flexible enough to consider.

Anax, Hardened in the Forge
Devotion might be an issue, but the ability to have some resiliency to boardwipes is something to consider.

Phoenix of Ash
Cheap way to get a few points of damage in the air while being a graveyard control tool with the Escape mechanic.

The First Iroan Games
A fair saga. An enchantment to consider if playing with delirium in the cubelet.

Atris, Oracle of Half-Truths
A fun mini-game with minor value add. You'll likely always take the pile with two cards.

Polukranos, Unchained
Probably one of the biggest bodies in most cubelets, but the fact that it shrinks with blocks/fighting makes it a contender for a cubelet

Shadowspear
A strong cheap artifact if your cubelet likes to play with cheap equipment.

Note on Adventure: If you play with adventure cards, you send them on an adventure and they return to you. Unlike the rest of the zones, you each send creatures on their own adventure.

Charming Prince
Two of the modes on Charming Prince are particularly strong for Cubelet and useful.

Brazen Borrower
Petty Theft is a neat tempo play and going on an adventure is your own thing, so the card is at your disposal for the rest of the game.

Murderous Rider
There are better removal spells, but this can be a nice budget inclusion.

Oathsworn Knight
Takes advantage of a player without a creature to get some damage in, otherwise, kind of a lackluster card

Rankle, Master of Pranks
Each player is very powerful as well and flying. If your cubelet is built in a way to handle these factors, then this is a neat inclusion.

Bonecrusher Giant
Fair card to consider. Nothing flashy or overpowered here.

Robber of the Rich
A neat way to catch up when you've fallen behind.

Oakhame Adversary
Plenty of green permanents to reduce the card's cost and the situation you can put the table in with deathtouch or drawing cards is interesting to consider.

The Royal Scions
The starting loyalty is a little high, but each of the effects, including the ultimate, are basic enough that the planeswalker is worth considering.

Stormfist Crusader
A unique way to accelerate the early game by rewarding more cards with some life loss.

Stonecoil Serpent
Great at any point on the curve and protection from multicolor is extremely relevant. Additionally, having another way to put an artifact into the graveyard for any delirium strategies.