This

If the Fantastic Four will ever come back to Marvel, it will be rather easy to incorporate them however, considerig their backstory can be easely injected at any time. With the last movie bombing and even with a sequel already being announced, I'd say if that one bombs too, 20th century fox will probably give back the license if it becomes a moneypit for them

The main problem with including the X-Men license is right now between Magneto (X-men) and his children (Avengers), and thus cannot legally mention each other (or the therm "Mutant" in Avengers, at least not in conjunction with superhumans), which would make any inclusion of the license down the line increasingly awkward when the characters are already more fleshed out.

We will sort of see how when they do Inhumans in a few years. Similar idea.

Infinity Gauntlet