Board Thread:General Discussion/@comment-2054358-20150809032033/@comment-9062114-20151130042756

But that's the thing, Fox has the opportunity to make more. The door to Marvel Studios is likely open for the X-Men. You're right, they're movies are definitely not failures. But the difference is, Age of Ultron can't team up with Jurassic World for more profits, but X-Men can team up with other Marvel properties. The studio ultimately wants to make as much profit as possible and spend as little at the same thing, teaming up with Marvel as I explained might do that in the future.

I think they are confirmed by Feige that the Defenders and Guardians of the Galaxy will crossover with The Avengers at some point http://www.cosmicbooknews.com/content/kevin-feige-hints-avengers-guardians-galaxy-crossover.

The point is, Marvel has a huge universe it can crossover and play with for years on end. If Fox really wants to not be a part of this, all they have is X-Men, or maybe FF, but that cannot crossover