Team Broccoli wrote:
This is assuming, of course, that 20th Century Fox doesn't own the rights to the characters.
I haven't read the agreements, but it's seemed to me like the movie studios have rights to the original license they sought plus anything derivative of that license. For example, Sony has Spider-man and thus also right to make a Venom movie (Venom having been created for and first appeared in Spider-man). Interestingly, this is why a Fantastic Four movie couldn't feature Namor. The Namor license is / was owned by Universal, and Namor is an independent creation from decades before the Fantastic Four.
Given that, Scarlet Witch and Quicksilver first appeared in X-men #4 (1964), and they were created for the X-men stories. I think 20th Century Fox has the rights. On the same token, Beast couldn't appear in Avengers even though he's just as notable for that team.
I think the Avengers movie will give some further insight. There have been rumors that...
We'll wait and see how they handle that potential situation; it could give some further clarification to how these movie rights shake things out.