I always said they should use the Utopia/5 Lights storyline.
Mutants have always existed, but maybe only let's say only 2000 scattered around the world (as opposed to 198 in the comics and hundred of thousands previously). The X-Men already existed, all of the previous events in X-History happened...Magneto origin, Prof X/X-Men origin, etc...there were once plenty of mutants but some unknown event happened and new mutants haven't been born in 20 years. And of course a lot were killed by Hate Groups.The second snap created the first new mutant born, and reactivated the X-gene in people so there is an instant exponential increase of mutants, so now people are being the bigots they are, because the mutants aren't heroes, they're just regular people trying to live their lives. The Utopia (or Genosha) angle was the gov't giving the X-Men an island to live on, and as long as they don't interfere with the world, they are left alone. The X-Men leave and set out to find these let's say 10 new mutants...and encounter the usual ish. On the other side, there is the Brotherhood, setting out to do the same....and the hate groups targeting them, and gov't standing idly about it. "But now is the time to stand up and fight...so pick a side"
IMO, they've already established that there were superheroes & powered-beings decades before Iron Man. It's easy to just have mutants already being there. The world wouldn't care being that there are so few around...and as long as they stayed out of ish. Out of sight, out of mind. The Utopia/Genosha angle gives a reason for those that want one, why they stayed out of ish. The X-Men don't usually get involved with the ish outside of Mutant-related events, and Avengers definitely never cared about Mutant plight, so keeping them separate always worked. And you get a reason for the X-Men to step back out into the world, with there being so many new mutants hunted/targeted/murdered Living while Mutant...that X-Men will not stand for it. Plus, you can keep whatever mutants you already had (Deadpool & Crew/Wolverine etc) also without over-explaining anything.
And they can even have the Phoenix Saga as already happening, and Jean disappeared X-years ago. The snapped awoke the Phoenix, which created the influx of mutants and Jean returns. Hell, it's the Search for Jean Grey...only they don't know it was Jean they were looking for.
ANYTHING involving bringing in the X-Men from another universe/multiverse is trash.