With Avengers: Endgame out in just two weeks, people are obviously trying to make up fan-service X-Men post credit scenes that they hope will show up in the movie. Unfortunately according to Kevin Feige it’s going to be quite a while before we see any of those characters show up in the MCU, and it could even be more than five years away.
In an interview with i09, Feige explains that they have a five-year Phase 4 plan and it doesn’t include the X-Men:
“It’s all just beginning and the five-year plan that we’ve been working on, we were working on before any of that was set. So really it’s much more, for us, less about specifics of when and where [the X-Men will appear] right now and more just the comfort factor and how nice it is that they’re home. That they’re all back. But it will be a very long time.”
The next MCU movies following Endgame and Spider-Man: Far From Home will be Black Widow and The Eternals, both of which are expected in 2020. In the comics, The Eternals deal with the Celestials; who are responsible for creating the X-Gene in the Marvel universe through genetic experimentation on primitive humans (the Kree created the Inhumans in the same way), so it’s always possible that the first hints at Mutants could appear in that movie, but with their actual MCU debut being years down the line.