The Question is a great idea. Completely forgot about him.
I was gonna go for a character I'd like to see as a villain : Equinox, who only existed for The Brave and the Bold if I'm not mistaken.
A god-like adversary obsessed with balance. Basically, if bad guys are winning, he comes helping to save the day, if everything is going great, he comes in to cause mayhem. Possibilities for great anything-stupid and for deep philosophy both.