According to a recent post by Badass Digest, there are rumors floating around that Avengers: Age of Ultron will introduce new superheroes as part of the superteam, and no – we’re not talking about Scarlett Witch and Quicksilver because those two are already confirmed to be part of the movie (although they start out on Team Ultron.) The rumors say that these are entirely new characters, including Ms. Marvel (which was already speculated to be part of Marvel’s plans for their cinematic universe.)
It is said that Age of Ultron will conclude with Captain America assembling a new team of superheroes consisting of characters old and new. As for what necessitated the formation of a new team, nobody knows yet but the studio has already promoted the film as a catalyst for big changes in their cinematic universe. The formation of a new team is certainly big enough.