Cowboys Clowns Secret Agents Geeks Cheerleaders Necromancers Tokyo Monsters
The real factor in making good expansions is variety in the deck and play styles, I have a feeling that the deck variation would run out before the theme ideas.
Deck styles I'd like to see would be
a no minion deck just full of actions that move stuff around and recycle stuff
a no action deck just pure minions
a deck that is centered around using other peoples powers for your own good
I do hope this gets a good few expansions for it as I love this game, so far I find, with just 4.5 plays under by belt, it has worked better with 3 just because it flows faster however 4 player games hopefully will speed up when people get to know the game.