Sure, there are still plenty of spells that are jokey, parasitic, or will otherwise probably not do all that much beyond Unstable s short-lived time as a draft set, but there are also a bunch of interesting cards that will make the leap into Commander and Cube
But this companion restriction is a bit too stringent
If you're creating a Cube, you could proxy the entire thing, or just individual cards that are hard to get a hold of
4 Nimbleclaw Adept ( LC, MC, ETB ) - This is an interesting ramp dragon, that allows you to untap two other permanents by tapping this, on a 3 mana 2/3
They are very popular in Final Fantasy, so there are plenty of Chocobo cards included in Magic: The Gathering's Universes Beyond crossover set with Final Fantasy