Any ideas on how they will be introduced?
I doubt the X-gene exists in current MCU.
Something must lead to triggering it.
Nah, still no idea or no hints from Marvel Studios on how it happens.
I'm just thinking of the start of the first X-Men movie, you could easily put that beginning scene after Endgame, change or add a few lines and it would make sense. The 5 year gap is a time period we haven't seen and you could easily say that mutants started appearing very frequently and that it could be related to the snappening and radiation and yadda yadda. The OG 5 and other mutants would be 14/15/16 at the snappening and their abilities are unlocked at that time. Jump forward 5 years, after everyone returns and they are 19/20/21, adults and ready to take on missions themselves.
But I feel mutants existed before the snappening, there just weren't very many and they hid from society, they aren't super heroes, just freaks with abilities or disfigurements. And the reason I feel that mutants have always been around is because of the comics history. Mutants, Inhumans, Kree, they've always been intertwined as part of the universes history and something they could easily introduce in the MCU. Namor and Apocalypse are 2 of the oldest mutants and so mutants have to have been around for a long time for them to exist.
And yes, anyone can randomly be a mutant, just like anyone can randomly be an Inhuman. And if two mutants have a baby, chances of them having a mutant baby is higher.