So. A week after Joss Whedon surprisingly confirms that X-Men/Avengers shared-custody kids Quicksilver and Scarlet Witch are being planned for "Avengers 2," apparently enabled by a contract loophole that lets them use these specific characters from the X-Men family (the movie rights to which are owned by Fox) so long as nobody says the words "Mutant" or "Magneto" (he's their dad); Fox and director Bryan Singer have now out-of-nowhere revealed that Evan Peters will be playing Quicksilver in the currently-shooting "X-Men: Days of Future Past."
This is interesting. Maybe.
First things first: Marvel/Disney would have absolutely zero problem with casting a different actor for the part in their movie, so the idea that this automatically means this guy will be showing up in "Avengers 2" is a non-starter to begin with. Besides, "DoFP" is a time-travel movie (supposedly involving lots of time-skipping and alternate-history in the service of cleaning up series continuity and presumably further deleting "Last Stand" and "Origins: Wolverine" from happening) so... yeah, likelihood that we'll see two different actors play a fast-running guy named Pietro in two different movies? Pretty damn high.
For the record: It also wouldn't surprise me at all for Quicksilver to have a really, really small walk-on role in "DoFP" - so small, in fact, that you'd think they perhaps very quickly wrote him into the movie once it was announced that he was to be an Avenger so they could benefit from the free-marketing of fan speculation. (Also, I expect they'll be casting young for the Avengers version of Wanda and Pietro; positioning them as the "unpredictable kid members" of a mostly adult-to-middle-age team.)
That having been said, the logistics of all this are kind of fascinating. My own pet theory (not supported, I stress, by any kind of special "insider info") is that "The Conversation" between Marvel/Disney and Fox about allowing The Avengers and X-Men to be seen holding hands in public is already taking place on some level (likely in the form of a childish staring-contest, but still). If nothing else, QS & SW are a strange choice for the first-announced new addition to The Avengers lineup, re: they aren't particularly popular, non-fans have never heard of them, their powersets aren't all that special and while it's true it gives the team one more woman it's still just two more white people on a team everyone seems to agree could use some diversity.
BUT! If they were allowed to be Mutants, with everything that entails? Suddenly it makes some kind of sense. Part 2 of a genre series is typically "the dark one," where things get complicated and awkward as the post-victory party winds down ("Yay! We blow'd up the Death Star!" "Crap, The Empire is resilient and this universe is actually pretty fractious and complex.") The sole non-upbeat undercurrent of "Avengers" was the idea that S.H.I.E.L.D. is willing and ready to play dirty as a response to a world "filling up" with superhumans, and it's important that the big "coming together" of the good guys happens in-tandem with them rejecting working "for" Nick Fury - even though he kinda sorta manipulated them into it, anyway.
If "Avengers 2" was to (or was able to, rather) explicitly say that the "filling up" of problematic individuals includes the "The Mutant Problem?" (They've already been floating the idea that Thanos won't be the "main" antagonist until Part 3, so there's also that.) Well, that's a really easy road to a darker scenario - the separation of The Avengers as the "good," accepted super-beings versus The Mutants as the "bad" ones people are worried about - and suddenly makes Wanda and Pietro interesting for the team.
The thing of it is, this is all on Marvel/Disney. Fox (and everyone else who owns Marvel movie-rights) would likely kill for their franchises to be declared even tangentially part of the Marvel movie-verse. "Avengers" was bigger than a hit, it was (and remains) a world-wide cultural phenomenon. Basically everyone saw it, the reception was overwhelmingly positive and it's absorption in the common language of pop-culture has been so immediate and all-encompassing that it's third-tier non-costumed supporting characters can now headline television series. If you're running a studio making superhero movies and there's some chance you could connect your movies to this juggernaut in the public eye, it'd be worth almost any price. Fox in particular should be salivating at the idea of being able to knock out a cheapjack X-Men tie-in and score a profitable weekend because it might be part of the "Avengers" story.
The trick of it is, while Fox (or Sony, if we're talking about Spider-Man) would probably meet any reasonable price to "share" the X-Men, it's Marvel/Disney that's in the position to A.) make the offer and B.) say yes or no; and there's really no (financial) reason for them to not just wait out the clock on the other studios running low on cash and just buying the franchises back wholesale so they don't have to share anything. The Avengers are, after all, already worth billions with "just" the six guys they already have - it's not like they stand to lose money if Spider-Man and Wolverine (lets be clear: Wolverine is the only reason the X-Men franchise is worth any money to any studio) aren't in the lineup.
In any case, it's a long way to "Avengers 2's" 2016 projected release date, and Marvel is (in)famous for making a lot of their movies up on the fly while shooting; so there's plenty of time for the situation to change on this. Right now it's a game of chicken, Fox saying "We're using Quicksilver first, so maybe start dealing with us or put up with fansites complaining about an 'actor switch' for your movie" and Marvel likely thinking "Yeah, because everyone was soooo mad that Edward Norton wasn't in Avengers;" but the math probably gets different if "The Wolverine" rescues it's franchise in a few months: Marvel is all about the money, and they know exactly how much of it a hairy forearm rising into the foreground in front of the assembled Avengers* and popping out claws to a familiar "snikt!" before a hard cut to black would be worth as the last shot of an "Avengers 2" trailer.
*Of course, like everyone else I'd LOVE to see the "Wolverine vs. all the Avengers" fight scene - with the caveat that Captain America ultimately knocks him on his ass, then gives him a hard time about how he remembers him being a lot tougher.