Cards to keep an eye on. These are cards that I think are either powerful or fun, and so would be interesting additions to the cube:

White:

Parallax Wave - Powerful card, flicker support, minor combo support, etc.
Prismatic Ending

Blue:

Rite of Replication / Mythos of Illuna - More interesting clone effects.
Complicate - Fun/interesting situational 3CMC counterspell.
Brazen Borrower - too expensive right now, but incredibly powerful.
Shelldock Isle - Incredibly powerful blue card.
Glen Elendra Archmage - Add Persist synergy.

Black:

Sedgemoor Witch - Riff on Young Pyromancer, but in black, with a better body. Very interesting card.
Nullpriest of Oblivion - 2/1 for 2 with relevant body that scales into late game.
Summon the Pack - "You win the game", at significant cost. Powerful & flashy.
Dark Confidant - Pricy, great card w. humans support.
Valki, God of Lies - Completely playable black card that doubles as a lategame mana sink in Rakdos decks.

Red:

Chaos Warp
Opportunistic Dragon - Sweet, anti-humans & artifacts.

Green:

Pest Infestation - Spell for tokens that also destroys things? Amazing rate.

Colourless:

Mazemind Tome - Powerful card advantage engine.
Nykthos, Shrine to Nyx

Gold/Hybrid:

Nicol Bolas, God-Pharaoh - Pushes Grixis in a fun direction.
Fiend Artisan - Powerful hybrid value creature.
Lurrus of the Dream-Den - Powerful hybrid value creature.
Jegantha, the Wellspring

MDFC's:
Jwari Disruption (Probably should be Censor?)
Glasspool Mimic - Needs to get in the cube ASAP. Great card and very versatile.
Sea Gate Restoration - Powerful late-game card. Only reason it is not run is due to price.
Emeria's Call - As above, but less cost prohibitive.
Malakir Rebirth
Sejiri Shelter
Spikefield Hazard
Tangled Florahedron / Turntimber Symbiosis - Both powerful green cards. Florahedron is the less exciting card. Turntimber is very good in cheaty decks, but a fine value card elsewhere. Even finding a three drop is good value.

MDFC spell/lands add a lot of versatility to decks, allowing them to find lands or spells. It makes gameplay far more reliable, and so helps prevent screw/flood - keeping games interesting, They are "boring" cards that lead to interesting games.