To be honest, based on Infinity War I'd say the MCU is already too crowded as it is; I wouldn't want mutants in it.

The FF at a stretch (boom-boom) could work, but then you end up adding Namor, Silver Surfer etc etc and where does it end?

I like the separate Universes, with potential for a dimensional crossover at best.

And I'm still not sure I want to see AF on screen.