We already have em. Both the PC and the Xbox have a max of 27 characters. You should play with Moon Knight a little more. His Xtreme is probably one of the best in the game, and the nunchuck power can take out a boss in a matter of seconds if you build it right.
As far as different characters, I can tell you that I'll never, ever do Dr. Doom. It's more personal reasons than anything else, but I think it's totally retarded that you can play as the main bad guy. I think Doom is an awesome character, and he's actually not bad to play as on the 360 /PC mod, but I personally will never do him. I'd like to do Hulk, in fact, Iammingy is working on a Xbox-compatible Hulk that I can use, but if I put him in I'd have to remove a character.
Cyclops I left out because he's been in like every X-Men game. I've been thinking about replacing Elektra, but honestly right now life is way to busy to work on MUA: SE. My next priority as far as mods go is finishing XML2: SE for the Xbox, but even that's on the back burner right now.