Ever since the purchase of 20th Century Fox (Now named 20th Century Studios) by Disney, two gripping questions have been in nerds' minds: Will Marvel finally incorporate the X-Men into the MCU, and how?
Like it or not, the answer to question #1 is almost assuredly a big fat "dollar-sign-tongue-emoji." But answering question #2 gets a little more complicated. The X-Men have canonically had a long and storied past, with mutants supposedly living among us for thousands of years (Though Apocalypse doesn't look a day over 999. Always moisturize.). It's hard to reconcile then how Thanos could show up to threaten all life on Earth only for Wolverine and Cyclops and even porcupine face guy all deciding to stay home and play Animal Crossing. (The 3DS version. It was 2018.) But one fan theory over on Reddit has attempted to reconcile that away, and it's not half bad.
"A convenient way to introduce mutants into the MCU is for them to first establish that mutants with powers have always existed in some capacity, but their prevalence has significantly increased since the snap. The snap caused radiation (as mentioned in Endgame) that elicited a massive epigenetic response in the surviving population with mutant genes."
Basically, Professor X, Jean Grey, and the gang have always been around, but before Endgame their powers were dormant. So thanks to a shot of energy from "The Snap," and maybe a couple of scoops of creatine for gainz, the X-Men are now ready to go off and make Marvel some money. But that still leaves the question of explaining this all away in the movies. For that, we look to this second fan theory here and the upcoming Eternals movie. (Slated for November 2020 or the soonest possible time we get out of hell.)
The theory states that cosmic beings known as the Celestials created three races: Eternals, Deviants, and Mutants. Mutants are essentially failed versions of Eternals and Deviants, which while superhuman, fail to possess the qualities of being immortal. It could be that, while The Eternals movie won't explicitly focus on mutants, it could lay the groundwork for us to see them later in the MCU, without Marvel having to make another five origin story movies just to get us to Wolverine swapping bodies with Peter Parker.
Top Image: Marvel Studios