As with Wolverine, the X-Men have by no means been treated poorly by Fox. In fact with New Mutants and X-Men: Dark Phoenix set to release in 2018, the franchise is in good shape. Yet neither fans nor Disney themselves could pass up the chance to see The X-Men and The Avengers share the screen, and the best way to bring them together is to adapt the 2012 Avengers Vs X-Men comic arc.
One of the biggest problems Marvel will have to overcome when introducing The X-Men is explaining where they have been throughout the MCU so far, and why mutants have been neither seen nor referenced even once.
With Doctor Strange introducing the idea of different dimensions into the MCU, the idea of having different realities isn't so farfetched as it once was. With The X-Men universe existing alongside the Avengers universe, there would have to be something big to bring the two realities together.
In the comics it is the Phoenix Force that causes this clash of the titans, however the MCU may want to change this for something already established in the universe, like Thanos' use of the reality stone, or Scarlet Witch's inability to control her powers. With the latter we could possibly see the House of M storyline integrated into AvX, similar to how Thor: Ragnarok integrated Planet Hulk.
How would you like to see the MCU integrate Fox characters? Share your thoughts down in the comments.