A five color mini cube!
They said it couldn’t be done, they said it shouldn’t be done!

A mini cube for 2-4 players, themes include:
-Shrines
-Coalation Victory/Door to Nothingness
-A surprising number of 1/1 flyers
-Domain
-Burn

For 4 players, I think you can consider 30 card decks, although there’s enough card draw in blue where decking could be an issue.