thegladiators From any canon, including movie-verse:
Thor and
The Hulk. My Cap is post-Fear Itself (unnecessarily long profile
here), but if you're not familiar with the comics, that's totally fine, since it would all be news to your character anyway. In fact, I think it would be pretty interesting to play comic!Cap against the movieverse!Avengers, because in his world, they've known each forever, but in their world, they've only just met. Some of them also have very different relationships/elaborate histories, which would create some conflict/confusion.
I'd also especially love to see: Sharon Carter,
Peter Parker (any canon), Falcon, Daredevil, Namor,
Wasp, Scarlet Witch, Spider-Woman, Ant-Man and
Wolverine.
Other X-Men characters would be great too (especially
Magneto, and Emma Frost). I'd personally prefer comic X-Men for the character link, since there's no connection between the X-Men films and the Avengers films.
The game could also use more villains! Marvel-wise:
Viper, Enchantress,
Doctor Doom, Elektra, any of the Hellfire Club, Mystique, and Venom.