SteelBird
Colonel
Marvel does not have the movie rights for X-Men, Fantastic-4 and until recently Spider-man. Marvel owns the original IP (comics) but when it comes to movies X-Men, Fantastic-4 rights go to Fox and Sony got Spider-man. Marvel/Disney and Sony came to an agreement with Spider-man and thats why you are seeing him in Civil-War, but X-Men for the moment no chance in hell. In fact Marvel can't even use the word "Mutant" in their movies. There was some loop-hole that allowed Marvel to use Scarlet-Witch and Quicksilver but they were prohibited to use the term "Mutant" hence the term "Enhanced" in the MCU. At one time it was rumored Marvel/Disney would reboot their entire comic line killing off x-men. MCU was going to challenge the X-Men with their own "Inhumans" and related series but thing seem to have cooled down a little bit.
I think it's only a matter of money. If Marvel agrees to share the profit, Fox will come in. The problem is if they put so many super heroes in a single movie, can they still make enough profit for everyone?
However, I am seeing a new movie is coming soon; X-Men: Apocalypse. going to see it? I'm not sure because I generally don't have that much time for cinema. Fox TV is going to show Captain America: age of Ultron on its channel soon. If you missed it at cinema, you can soon enjoy it at home.