First of all, congratulations on your first post.
Secondly, I hope Marvel and Feige plays it reaaaaally slow when it comes to incorporating the X-men into the MCU. I mean, first of all, obviously they should not cross the Fox X-Men Universe with the MCU, and I don't think anyone is thinking that. When the X-Men finally do enter the MCU, if this deal does indeed happen, it will obviously be with rebooted X-Men characters portrayed by new actors.
Now, since the MCU has such a huge cast of characters already, and I really hope they get the rights back to all the Spider-man characters soon too, they can't just pile them all in there at once. And with the X-Men it basically doubles the amount of characters involved. Also, it would kind of make "normal" superheroes pale when every other teenager suddenly has superhuman powers as they hit puberty and their mutant genes materializes. Not to mention that they can't just throw them in there, they have to be added in a way that is believable. I mean, if the mutants have been around forever why are we just hearing about them now?
So, here's the incredibly slow play I hope that they will opt for as they incorporate them into the MCU:
Step 1) Because the Scarlet-Witch and Quicksilver got their powers from the Infinity Stone that was in Loki's staff, they should make the X-Men origin story for the MCU like this: When the Avengers defeats Thanos they should also destroy the Infinity Stones, and they should do it on Earth. The Infinity Stones' destruction should cause their essence to spread across the atmosphere of the Earth as pollution that will ultimately affect our genes and have some of our future children develop the X-gene. This would act as a setup way in advance, and basically retroactively name the Scarlet-Witch and Quicksilver the original Mutants in the MCU.
Step 2) Continue with the post-Phases "second period" of the MCU as already planned. Remember, Kevin Feige has said they already have 20 films planned for this "second period", with Spider-Man "Homecoming 2" and Guardians of the Galaxy 3 being the only two we know about so far. So they should continue with their plans and
not do the X-Men at all for the next 10-15 years after Phase 3 concludes with the fourth Avengers movie.
In the meantime, the people behind the current Fox X-Men cinematic universe will continue to produce their movies and television series while being owned by Disney, independent of Kevin Feige's people, with the added benefit of knowing that their universe will have to conclude within those 10-15 years,
before the X-Men characters are rebooted for entry into the MCU.
Step 3) Introduce Charles Xavier, Magneto and a limited number of the original set of X-Men characters as teenagers as they discover their powers, and include them in the MCU on a level on par with the level of involvement of the Guardians of the Galaxy today. Just gently put them in the mix of the grown-up Spider-Man, whoever is Captain America by this time, etc.
Step 4) Then, in the 2040's (be patient!) let the X-Men virtually take over the MCU as Charles Xavier starts his school and Magneto transitions from having been a grey Loki-like character in "Step 3" to being an outright villain set to conquer mankind for the sake of Mutant kind, and unleash the full roster of X-Men characters as this conflict puts on heat.
Step 5) Kevin Feige's retires in his 70's and they reboot the whole thing. It will probobly suck, but the CGI will be great!