I don't mind them doing crossovers or making excuses why the teams didn't help each other, the shyt happens all the time in the comics. My issue is if they bring the X-Men to the current MCU, like others, I see no logical way to explain where all the mutants were this whole time. It's easy to come up with the X-Men themselves not being around but not mutantkind in general.
You gotta retcon their whole story since no global oppression of mutantkind has ever existed in this universe, seeing as there ARE NO mutants in this universe. Their whole reason for existence doesn't exist so there's literally no reason to form an X-Team, build Sentinels, Morlocks hiding, etc. The tension & oppression needed for them can't just come out the blue, like "Oh, these people were having a civil war in Genosha this whole time and are moving around the world now. We just realized we should be scared."