I would like to see him as a reluctant participant in th Avengers... lured in with promises of total control over any GAMMA experiments that he wants (in order to understand everything about the transformations, and to even find a cure)... and Betty has been recruited to the SHIELD scientific team, so whe will be there.
Then there is an scene in film, where he loses control and dukes it out with the team... He regains control and transforms back... "he says, no one can help me, I need to do this on my own..." he leaves. Of course Fury keeps an eye on him, and when the final battle culminates, Fury tells Betty that he knows Bruce's whereabouts and she needs to convince him to come back... He shows up, and helps save the day.
This opens things up for him to have another solo film down the road.