I don't think we'll see X-Men or Fantastic Four from Marvel for a few years. They will most likely want the sour taste of the Fox X-Men and FF to wear off a bit before introducing mutants and the X-Men in the MCU.
If I were them, I'd probably plant the seeds in a few movies about mutant before introducing the X-Men. Since the movies for 2019 are already filmed it would probably be difficult then.
In the 2020 movies, I would start rumors of mutants but not necessary see any
In the 2021 you could slowly start introducing a few in existing franchises, either as villains, or in the same way they did with Black Panther and Civil War.
Then in 2022 you do a few movie, and then introduce X-Men as a group by the end 2022, or in 2023. You do it slow and methodically like they did with the Avengers, you don't rush it like DC did with Justice League.
Marvel can afford to take it's time with this...they already have solid franchises to put 3 movies a year, I doubt they want to make much more than that to avoid super-hero fatigue, and stretching themselves too thin and let quality go down.
I think we're more likely to see Dr. Doom, Silver Surfer, Galactus and other stuff from the FF rights before we see X-Men to be honest.
It's awesome news though.