I think it's going to be Loki manipulating HYDRA and certain elements of SHIELD.
At some point, they have to use Big Green to stop some ginormo menace because Thor's been knocked out or fighting somewhere else, Iron Man's undergoing emergency repairs, etc.. So Big Green comes in, saves the day and...
Aw, crap. He's still pissed.
So it's up to The Avengers to step in and stop The Hulk before he destroys the eastern seaboard.