Re: Changing the roster for Avengers 2?
In terms of the properties still in the possession of other studios, we can already see the attempts at creating their own "Universe" films with the announcement of a Venom film. I seem to remember the X-franchise was supposed to be much more fleshed out, too, with more "Origins" movies. The problem was they didn't have the right people working on those movies, but now? I kinda feel they do.
A massive cross-over film would be difficult to wrangle, and would teeter dangerously towards pointless fan-service, like The Last Stand including Multiple Man for no goddamn reason, and Phoenix - arguably the greatest character in X-Men comics - becoming little more than a foot-note pretending to be a catalyst.
Additionally, these films aren't made entirely for the hardcore comic book fans, they are also heavily marketed to people who have never read a comic book in their lives, so too many nods to characters (such as the - albeit cool - Black Panther cameo someone mentioned earlier) might leave them scratching their heads.
But hey, I'm not saying it couldn't be done - focus on a key group of five or six heroes, with others providing supporting roles. A lot of characterisation would need to be handled through action sequences, I think.