No way. Feige will throw that shit out and start over. I read a cool idea the other day, that they should introduce everyone via Doom. Have what's left of the Avengers have to fight him for some reason, and when they beat him they break some kind of machine that's been keeping the Fantastic Four from existing in the current timeline, and now they're back.
That or however they un-snap folks causes the X-gene, and suddenly mutants.
I don't think they should tie in the FOX timelines at all.