Re: Changing the roster for Avengers 2?
It would be beneficial for both studios to include Spider-Man in the continuity, even if it was for a side role. Oscorp tower was going to be in The Avengers until the last minute. That's a right of Sony's film, so they played ball on some level. By putting that tower in, they were accepting that Spider-Man existed within the continuity.
A cameo isn't beyond the question. Clearly in this case.