I can already predict what it is.
It's already been set up with the Avengers 2 movie.
Mutants will go away.
But the new term for them will be Inhumans.
Just as Scarlet Witch and Quicksilver are being called Inhumans in the Avengers 2 movie.
I assume, this is so that Marvel can create an "X-Men" movie despite Fox currently having the rights - since they won't be mutants anymore.
Same reason why Quicksilver can appear in X-Men Days Of Future Past, as well as Avengers 2.
They're going to say it's different characters, in a round about way.
That's my theory and I am sticking to it.