In what might be the first solid and official answer to the title question of this thread, Kevin Feige might have just let slip the actual roster for Avengers 2:
"It's still early days," he said, "but I told [Whedon] after reading the first sketch outline he delivered — and there's a lot of work still to do, he has to sit down and turn this into a movie — but there are already seven things in there that are worth the price of admission alone."
Don't bother asking Feige what those seven things are — believe me, I tried; the man is a vault. But Feige insists that some of "the seven" won't make sense to fans until the upcoming Marvel movies like "Captain America: The Winter Soldier" hit theaters.
Sounds very, very much like Feige saying that Falcon joins the team, as has long been rumored, and that he's the only addition for Avengers 2.