Re: Changing the roster for Avengers 2?
Right. The comics roster rotates, but usually not arbitrarily - the writer introduces their team, and writes several arcs for that team, with a character occasionally getting a big flowery exit or a new character or two being added as the story progresses. Rarely do we see characters being unceremoniously dumped from the team with no real reason given. (Well, actually that happened quite a few times during Bendis' run, but that was one of the problems with his writing.)
I love at least a dozen or so Avengers who I know will never be in the movies; that doesn't mean I would want Black Widow or Hulk to simply not appear in Avengers 2 just so Black Knight or Monica Rambeau could show up.