Looking at this thread makes me even happier that I'm getting a Wii soon. Hope that there's a way to get more than 29 characters.
Anyway, here's who I voted for.
If there's no way over the 29 limit, then I would replace Nick Fury with Punisher and Colossus with Hulk and have Gambit, Hawkeye, Iron Fist and Captain Marvel in the PSP slots.
Also, I would suggest using the Eddie Brock Venom skin as Venom's default skin, not Gargan. Because Mac Gargan is still the Scorpion in this game, and it makes more sense for Eddie to join the alliance than Gargan.