Oh i honestly haven't watched the latest X-Men movies. Fox ran it into the ground and then set fire to it. I'm more a fan from the old cartoons and comics.
Seeing how they'll include the mutants will be interesting. In theory they could just have always been a thing and just never been brought up, seeing they've not been relavent.
The Eternals movie also serving to introduce the X-Men would be a good idea. Seeing they share a related orgin in the comics, if i remember things properly.
I usually don't care for Mysterio, but the version in the movie was really good. Giving him a whole team for the special effects, was a nice tough. I wouldn't be surprised, if he somehow survived and reappears using a different alias.