Marvel has released the teaser trailer for Marvel Ultimate Alliance 2: Fusion and you should do yourself a favor and check it out.
While Marvel had this to say about the game:
"Looking for details? Characters? Storyline? Gameplay? Release Date? Sorry, friends, that'll all have to wait! Enjoy the teaser trailer and keep checking Marvel.com over the coming months for more "Marvel Ultimate Alliance 2: Fusion" news and goodies!"
Activision has been a little more open with some information and revealed that there will be 24 playable characters in which Iron Man, Fantastic 4, Venom, Green Goblin, Spider-Man, Hulk, Captain America, and Wolverine; and since the plot seems to revolve around a global assault around Latvia so you can assume Dr. Doom will be there.
From what I can tell from the teaser I believe the "Fusion" subtitle comes from a new ability to combine character's powers for new moves and upgrades.
The first game kicked a lot of ass and I am looking forward to the sequel, which according to a few unconfirmed sources will contain elements from Civil War. More information to come over the next few months as it is released!