Phyrexia: All Will Be One

Annex Sentry
An interesting Fiend Hunter that would introduce poison counters into the cubelet if you don't play it already.

Norn's Wellspring
​Get a little value out of your stuff dying and scry is one of the best things you can do in Cubelet.

Phyrexian Vindicator
Kind of gnarly. Might be too good, but if you like the higher power stuff, this meets it.

Skrelv, Defector Mite
An option if Mother of Runes or Giver of Runes isn't your thing. Toxic is also kind of funny to add to a cubelet. Nice for a one-drop cubelet.

Experimental Augury
If you have a bunch of counters in your cubelet, proliferate is always nice. We'll take improved cards any day.

Glistener Seer
Scry? Good. One-drop? Great for a one-drop cubelet. Doesn't have defender? ooo weee!

Jace, the Perfected Mind
Listen, milling the library in Cubelet...kind of stupid (not something you should actively do). Drawing three cards because you share the graveyard so twenty cards is relatively easy? Yea, that's kind of good.

Minor Misstep
A great counter spell for a one-drop cubelet! Nearly counters everything! (Curse you Killing Wave)

Unctus, Grand Metatect
Pay some life, attack, loot? Sign me up!

Sheoldred's Edict
What an upgrade!

Blazing Crescendo
Neat combat trick. Not normally something you see in a cubelet, but the pseudo-draw you get makes this playable.

Gleeful Demolition
Always like a cheap shatter effect. This has upside if you hit yourself!

Bloated Contaminator
Beefy and threatening, love to see it,

Cankerbloom
Options, lots of options. Could combine a couple cards in your cubelet, giving you options for other activities.
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Conduit of Worlds
The first line of text doesn't matter, really it is letting you play with the graveyard if you want.

Evolving Adaptive
Love the Cloudfin Raptors in the game.

Green Sun's Twilight
You're not searching the library. It's like a really neat lottery ticket with a big payoff late game.

Charforger
Interesting. Very Interesting.

Glissa, Sunslayer
Love all the options on Glissa.

Kaito, Dancing Shadow
Great value card, nothing too powerful.

Venser, Corpse Puppet
I just think this is a neat card, would definitely fit in a cubelet.

The Skullbomb Cycle
They are fine and can be applied in a few different cubelets (one-drop, pauper, artifacts matter, etc.)

The Filigree Sylex
Seemingly innocent, could maybe make a cool game variant using it, similar to Strixhaven Stadium.