To further expound on my previous post, nothing has happened in the MCU that completely negates the possibility of the X-men also existing. Is it far fetched? You could argue that, but there are not any explicit reasons the two franchises could never ever intersect.
"Well where were the X-men when the Avengers fought in New York?"
I don't know, where was Captain America when the president of the united states was kidnapped in Iron Man 3? The MCU is already set up so that each hero pretty much lives in isolation, it's one of those things that you just have to accept, same as the comics.
"Well why haven't they mentioned mutants in the MCU before?"
Why haven't they mentioned Harry Truman? Or Wendys? Or lava lamps? Just because something is not ever pointed out and discussed doesn't mean that it isn't there somewhere off the screen.
"Well in the X-men films-"
Let me stop you right there, it does not matter what has happened in the X-film series because Fox has blatantly contradicted their own continuity too many times to count so one more thing to the list wouldn't matter.
Now, am I arguing for an MCU/X-men cross over? Of course not, but I'm laying out clearly why that makes just as much sense as crossing them over with the FF.
You can talk all day about how "with the right execution" it could be done, but as I've said before, I'm sure that same thing was said when the idea of a Freddy v Jason or Alien v Predator movie was floating around.
It makes no sense whatsoever to see some of you adamantly reject the idea of the X-men or Spider-man crossing over with the MCU, and then talk about how much you'd love to see the FF and the X-men crossover. None whatsoever.