You mean just like batman/superman is half a justice league movie?
I get what your saying but the difference is that because the avengers has already happened, there are relationships between the characters that are already established. Now those relationships can be developed even more. Hell, maybe even lead it into the next avengers movie for that phase.
I am with pairing iron man and the hulk together formsure. I think you could possibly put iron man and ant-man together and have it work. Pair a lesser known character with a publicly well-received/well-liked character like iron man, could give ant-man a boost in the eyes of the general audience. I also wouldn't mind seeing maybe a pairing of Thor and hulk. Watching a movie where they're constantly trying to one up each other. Who wouldn't want to watch that. Maybe do what the comics are doing now and pair iron man with the Guardians of the Galaxy.