You know what character would be interesting to see made playable from the more obscure Marvel roster? Howard the Duck.
I mean hey, someone's managed to make Rocket Raccoon work within the limits of Marvel Ultimate Alliance. So it wouldn't be completely out of left-field, for a talking duck who knows kung-fu and a bit of magic to appear in the game too
Just putting it out there as a potential idea, considering there aren't many Marvel characters left now, who haven't been made playable after nearly 10 years, since this game came out in '06.
EDIT: Man-Thing could be another interesting one to play around with, so long as you know what you're doing with him.