Much of what is being discussed around Avengers: Endgame involves the introduction of the Quantum Realm, possible time travel, and the demise of some of our heroes. We've seen tech, chemistry, skills, robots, aliens and magic all at play in the MCU. We've also seen near omnipotence. And omnipotence won.
The conclusion to Avengers: Endgame won't likely be the Avengers slugging it out with Thanos, similar to what they did in Avengers: Infinity War. To what end? To convince him to snap back everyone lost back into existence? That would require some serious leverage against somebody that sacrificed their daughter for their cause. No, I propose that Avengers: Endgame will be a McGuffin chase. It's possible Thanos won't even factor into it. (By the way, what have you heard about Josh Brolin filming in Endgame?). It has been suggested that there will be other evils involved. That's one of the true surprises in Avengers: Endgame.
But what of the McGuffin? What is it in this case? (If you don't know what a McGuffin is, go visit Wikipedia)
There will be a Quantum Realm/Time Travel journey that will enable a key change in the MCU to prevent Thanos' snap. It could be a change to one or more of the Infinity Stone origins, or a key character, maybe even Thanos himself. Whatever it is, that's the Endgame. We know things will be undone (Far From Home are we?). What we presume is that things will be normal once again after we finish reading the credits. But ... What if that key change, at a time in the past to undo the future, also resulted with a genetic change in the MCU.
Hey, we know that the Marvel machine will continue to churn out movies based on the Marvel Cinematic Universe. Certainly one of the challenges is the acknowledging that the MCU is not the complete Marvel Universe. By completing the MCU, the opportunity for fun & profit will expand - providing of course that the movie-going public doesn't tire of CBMs.
So there it is. Undo Thanos' snap and introduce the X-gene in the process. We know it has to be done in the past and not the present because otherwise all your X-heroes will be born next year. I hardly think Disney will wait for them to grow up! This all fits in with what we know of Endgame. The piece about a genetic alteration might be a side effect or the actual Endgame. We shall see. But it does allow for the X-men and even the Fantastic Four to already exist in the present day MCU. Imagine pulling out from the Stark/Avengers tower and seeing the Baxter Building close by. What a reveal that would be. No casting required ... but so telling.