Where to begin? Hmmm. Lets start with Marvel Studios announced that in the current script for The Avengers 2, Quicksilver and Scarlet Witch were going to be joining the cast. Thats cool and all, but they problems were they are Magneto's kids and thats were it gets legal fuzzy.
Does Marvel and Fox have an agreement to use the characters 2 separate films? Or did Fox just pull this out of nowhere to really stick it Marvel about stealing two of their characters? You don't see Captain America aka Steve Rodgers showing up in The Wolverine without the Stars and Stripes.
HitFix reports "we're going to see is a legally-negotiated stand-off in which we'll get two totally different versions of one character. While (Fox and Marvel) may act like things are amicable in public, HitFix sources say otherwise." They add, "The way the arrangement works, Marvel Studios cannot refer to the character as a 'mutant,' and they can't mention Magneto at all. Likewise, Fox can't make any reference to The Avengers in their film. That means they'll have to either explain his powers in a different way in The Avengers 2, or they'll just have to make no mention of his origin at all."
More info to come in due time when this all gets figured out


