An alternate universe could do the job. Inhumans and mutants in the same universe means confusion. A lot of confusion.
If Marvel could establish a separate universe especially for the X-Men, they could manage the storylines much easily causing less confusion.
Now coming to the public demand:
The ULTIMATE crossover; the geeky dream of every comics fan.
I have nothing against a crossover. But considering the condition of the mainstream MCU after Infinity War. The highly celebrated members of the Avengers may retire from their superhero business. The contracts of the actors are already expiring. Evans has only one more movie to go and RDJ has already been in a lot of movies. The new Avengers will include lesser known superheroes and if there is a crossover, I doubt the return of the stars and the possibility of a crossover
But yes some fan theories do make sense and there are possibilities of adjusting the X-Men in the mainstream MCU.
P.S.: Considering the fan theories including the usage of Infinity stones to restore the damage done after IW. I don't think Marvel wants to spoil their only dark movie with a happy ending. They've already spoilt the seriousness of a previous movie by restoring the damage done(on a smaller scale though)