Re: Quicksilver's hair - how should it look?
While fox might own the film right for "mutants", they can't own the rights to a genetic mutation. You can't own the rights to something that isn't copy writeable. They can easily say they were born with their powers. They just can't outright talk about being Mutants or being part of the Brotherhood of Mutants, or mention Magneto, etc.
“Whenever you see somebody crossing over to something it gives it pop. Like Murder She Wrote and Magnum [P.I.] — great crossover. I got excited.” Joss Whedon on Spider-man joining the Avengers.