There was a thought for a very long time that Marvel would never be able to have Spider-Man in their movies. However, mountains moved, Sony and Marvel came to an agreement and we ended up getting him in Captain America: Civil War.
This has lead people to think about the possibility of Fox and Marvel coming to a similar agreement for X-Men and Fantastic Four. However, according to this answer Kevin Feige gave to Collider, I wouldn’t hold my breath. On the possibility of a deal with Fox, he said:
Well, it’s the same as it has always been. I don’t want to attribute it as a frozen wasteland or anything like that.
We all get along and I see them occasionally but there’s no change in any dynamic.
Personally, I think this might stretch the Marvel Cinematic Universe past breaking point anyways. The X-Men would be really cool to see on screen with the Avengers, but that universe is already so big on its own with a really storied history, I just can’t see it working. Bringing in mutants to the MCU now equally feels complex, given its history, especially with Scarlet Witch and Quicksilver.