I wouldn't mind having some characters powered down (like Thor, Hulk, Surfer, Blackbolt, Captain Marvel) so that they weren't too uber powerful compared to the rest, as long as the rest of charcters were less powered down in other areas, like superstrong characters should have stronger melee than ones of regular strength.
I like the anger bar idea for Hulk. I think they should incorporate more ideas like this into the characters to help diversify the gameplay. Superheroes aren't all the same. Characters like Cloak and Dagger would be cool beacuse you could play as one at a time and switch between them.
Blackbolt would be cool if his devistating voice power could only be able to be used once in a while. It would basically kill everyone, but take a long time to refill.
Dazzler could be neat because she could have a sound meter. When it's full her moves are stronger and as she uses them it depletes, you'd have to recharge it a little if you wanted her powers to be at their highest level.
Anything they could do to add an extra layer of strategy into the game would be great.