While there is technically a "No Dibs" rule, these are the characters that players have shown interest in so far.

Spider-Man
Hulk
The Wasp
Winter Soldier
The Fantastic 4 (Johnny, Sue, and Reed?)
Moon Knight
Ms. America (Invaders)
Wonderman
Speed (I think)

Therefore, there is plenty of room for some Avengers (Iron Man, Captain America, etc.)

Edit: I also have interest in Hawkeye and Cyclops, but I am waiting for the game to start before I pick up more characters.