my bet would be some kind of blend of the plot Avengers #1 and the Super-Human arc of The Ultimates.
Loki manipulates Hulk into attacking New York and the Ultimates, sorry Avengershave to beat him down as he tears up mid-town in a major slug-fest.
Sounds likely enough, but in the astronomically unlikely event that Whedon goes the Mark Millar route with the Hulk's characterisation, I may disavow the movie and leave. I think that there'll be a more character-based and story-based vibe to this movie than Millar's shock-tactic approach.
Captain Craig wrote:
The smoothest, easiest and best reason to get the cinematic Avengers together can be done very similar to if not exactly like it's 4-color counterpart did it back in the '60s.
Sure, why not? I can't see Thor being a minor player, as he kind of brings mythology and adventure to the party. Cap has to be integrated in some significant way, though.