It was already a big deal when Joss Whedon explicitly confirmed that two mutants were already in the line-up for The Avengers 2. While the terms of Fox’s current deal with Marvel states that Marvel & Co. could not refer to the characters as “mutants” in said superhero team-up sequel (nor could their heritage as 
Magneto’s progeny be mentioned), it seemed as if we were finally going to see signs of Marvel broadening their universe past what they currently control, if only subtly.
Well shit just got much more real.
Word is Bryan Singer has come up with a sequence for X-Men: Days Of Future Past that requires the involvement of a super-speedy character. Evan Peters has just been cast in the role of Quicksilver, which means a key new character in Whedon’s Avengers line-up will show up in an X-Men movie as a side character first. According to Hitfix this means the lawyers are definitely getting called, even if it’s just so that some sort of truce can be put in place. That said, nobody is happy about it apparently.
“…what we’re going to see is a legally-negotiated stand-off in which we’ll get two totally different versions of one character. While they may act like things are amicable in public, HitFix sources say otherwise.”
While it’s probably safe to say most geeks sympathies sit with Marvel at this point, the fact is that the current deal gives Fox much more creative leeway with the character, since ultimately his ties are more explicit with the X-universe. They also shoot first, so they get to set the stage. I 
imagine this is all a great big headache for Mr. Whedon, who already had the unenviable position of establishing characters whose origins are legally mandated to remain mysterious.
I suspect the clumsy outcome of there being two versions of the same guy in two different franchises is where we’re going to end up seeing. I also don’t suspect most consumer will really care that much- anyone who notices enough to look it up will have their answer to the confusion readily enough. That’s not even considering how utterly disposable every mutant appearance in the X franchise has been, save for the absolute key players. But with Saoirse Ronan already rumored to play Quicksilver’s sister Scarlet Witch in Avengers 2, it seems Whedon’s line-up is moving forward, even if Singer is marking territory first.
That said, if Quicksilver ends up they key to some barnburner sequence in 2014, that’s going to crimp Whedon’s style in 2015 when he debuts his own Quicksilver. And while we’ve heard that Sony might be up for slowly looping their Spider-Man franchise into Marvel’s universe, it does not appear that Fox has similar ambitions.
Which version of the character are you most excited to see?