Well, captain america will obviously be against Red Skull. Don't know too much about Hulk villains, although he did fight Thor in one of the 70s movies. I doubt it will be the Abomination again tbh for some reason. They gots to mix it up.
Basically, Samuel L Jackson is going to be Nick Fury, i.e. end of Iron Man movie. But Marvel have a Captain America, Thor and Wolverine movie franchise all planned. THEN we will see the avengers made from Hulk, Thor, Iron Man, Wolverine, Captain America. Although i'm disappointed that Spiderman won't be included as he was later picked up :(
http://en.wikipedia.org/wiki/Avengers_(comics)