I don't understand why all of you are suggesting that Hulk should appear in a Avengers film.
I think that his character will take most of the time and a plot of the film, and still we would have characters that weren't introduced before, unlike Iron Man and Captain America, or even Thor, characters like Giant Man, Wasp, Vision, maybe Hawkeye... The Hulk is a very demanding character, which will always take the climax in his own hands. I think the film will deal with the introduction of some less popular characters and the team's assembly and defeating a threat like Kang the Conqueror, Ultron, or maybe the best choice for the first film would be the Masters of Evil. Maybe the Hulk would fit in a future installment of the Avengers, but I somehow don't see him in the first one.
There is one other thing I would like to point out. Is this speculation about Hulk being in a Avengers film is formed from the last scene of the film, the scene with Tony Stark? As I remember Stark approaches General Ross about the formation of the team. So where all this speculation comes that Hulk might be in it. Ok... he might, but a more logical thing is that general Ross will play a significant role in the film. My guess is that Tony Stark comes to Ross to find out about the super soldier serum and Steve Rogers aka Captain America, and not the Hulk. Even Hulk's plot was about the serum so why not take that possibility?
I personally would like to see a sequel to the Incredible Hulk, because if that doesn't happen the character is dead for good. There will not be a remake for another 20 years. The film was great, not amazing, but great, and that's because it was intended to have a sequel. The story was not finished. Even Iron Man feels more complete a film than Hulk does. Somehow the film doesn't explore all the potentials and possibilities. There is even a scene with the Leader's origin... where would you go with that? Also in Avengers? I think not. I think that they are releasing an extended edition of the film, which will make more money for a sequel and even bring back Norton's trust. But I also don't see how he doesn't have interest in a sequel, as was stated by many. He just doesn't know the future of the franchise. I think that there will be another Hulk film, maybe a last one, that will put a conclusion to the story, but it will happen eventually some time.