Long rumored but now seemingly confirmed by Joss Whedon to IGN, joining the team (or at least in the film to some capacity) will be Wanda and Pietro Maximoff, better known as The Scarlet Witch and Quicksilver. He's basically The Flash if he was a Eurotrashy douchebag, she can manipulate probability - i.e. she can make pretty-much anything happen through "magic" handwaved with psuedo-science. They're fraternal twins, originally villains who reformed and joined The Avengers alongside fellow ex-baddie Hawkeye during the team's first major overhaul (everyone quit but Captain America, leaving him to train a new team of ex-villains.)
Here's why this is kind of a big deal: Quicksilver and Scarlet Witch, in the comcis, are Mutants. Supposedly, "all" of Marvel's Mutants are owned as part of the "X-Men Family" by Fox; so they can't show up in any non-Fox Marvel films. Normally, that shouldn't be a problem: "Mutant" in the Marvel Universe essentially means anyone who is a human born with an extra-human sense or ability, so it would not be tremendously difficult to augment them into some other variation on "born like this" (or just give them new origins) for "Avengers 2."
But these aren't just any two Mutants: Despite being overall more-associated with the Avengers franchise (which is why they can be in this at all, Mutant or not) Quicksilver and Scarlet Witch started out as members of The Brotherhood of Evil Mutants... and they're Magneto's children.
Now, sure - that can be written out just as easily as having them be capital-M Mutants, but it also negates most of what would make them novel as members of an ostensible good-guy team. And they're not exactly popular or novel enough for me to imagine Marvel insisting on their presence - nobody really cares about Quicksilver and there are many more-popular Marvel heroines than Scarlet Witch - so Joss Whedon or someone else on the creative side wanted them specifically for this. That's interesting.
Here's the thing: Fox would probably do anything (short of relinquishing the X-rights themselves) to have "their" X-Men franchise be "officially" connected to "Avengers," even tangentially. Suddenly, any random Mutant movie they crap out becomes much close to must-see because "hey, we might need to have seen this for the next 'Avengers' movie!" So the question isn't whether or not Fox would "go for" some kind of shared-custody of the Mutant characters (they probably would,) nor if Fox and Disney/Marvel have already talked about this (you'd best believe they have - everyone who owns a Marvel franchise has been talking about how to get in on the action, FX scheduling is supposedly the only reason you didn't see "Amazing Spider-Man's" OsCorp building in "Avengers.") The only question is whether Marvel is willing to "work something out" or would rather hold back until Fox has a bad year and needs to sell the X-characters back altogether.
I have a suspicion that there's another shoe or three yet to drop on this, especially since there's a LONG time between now and "Avengers 2." But for now, Fox has two "X-Men" movies set for this year alone, and if they're both hits that gives them a slightly stronger bargaining position assuming any bargaining is actively going on (I suspect there is, but you never know.)
