Joss Whedon has publicly hinted at a brother/sister combo appearing in The Avengers 2, and EW believes they can confirm their identities: Scarlet Witch and Quicksilver, the mutants.
Lest you think this is a sign of studio crossover of Marvel characters, these two are actually in licensing limbo right now- they remain unaffiliated until one studio or another uses them and lays claim. Though Whedon notes that script’s roster could change long before shooting begins, it appears Marvel will bring these two particular heroes home. NOTE: Shawn Lealos pointed out to me that Fiege has explained this arrangement in more detail, and they could appear in both studio’s films as long as they don’t reference the other universe’s specific details.
Quicksilver is an infamous villain turned hero, having once been a part of the Brotherhood of Evil Mutants. He and his sister are actually the spawn of Magneto, so these aren’t throwaway characters- both have been integral parts of both The Avengers and various X-teams. Scarlet Witch is a heavily magic-based character that bends reality, while Quicksilver is essentially a Marvel version of the Flash. The Avengers 2 will definitely be doubling down on humans with outright superpowers- kind of a new thing when you think about it.
What this does do is explicitly bring mutants into the Marvel Studio universe, which the film
will either has to ignore, or opens up some weird continuity legal doors. Interesting watershed, that.