We shouldn't be expecting the Marvel Fox characters to join the MCU soon; at best we have to wait five more years before they officially arrive, but Marvel Studios has already started thinking about how to introduce the characters into the universe.
Talking to Fandango, Marvel Studios head Kevin Feige explains:
"It's still early days, but it's been a fun exercise, and it's one, by the way, that we've been doing for years. Every development meeting starts with cool ideas and fun ideas, and our wheels are always turning in terms of what if… to use a Marvel publishing term. What if we did this? What if we did that? What if we had access to such and such characters? That's how Spider-Man: Homecoming came together in the first place, and it's fun to now be in this position with the Fox characters, too, because if we come up with a great ‘What If' we can actually do it."
Though introducing Spider-Man to the MCU is one thing, it's going to be completely different when it comes to introducing something as massive as mutants to the MCU. With wisened characters like Professor X and Magneto, it wouldn't make sense to just have mutants suddenly start popping up out of nowhere; at best, I think the remedy for this is a multi-verse hiccup and we suddenly have a world where mutants have been existing for a while.
As for the Fantastic Four, which I think has a better chance of coming out first, I think it would be cool to introduce them in a period film, seeing as they're famous for being "Marvel's First Family." Maybe we could have the four introduced as a family in the 60s who somehow travels in time and ends up in the present day (like Captain America).
My other idea for an introduction to the Fantastic Four is that they're the ones who ended up buying Stark Tower in Homecoming, and they turn it into their headquarters—the Baxter Building.
I'm just spit-balling here, and I'm sure the writers at Marvel have better ideas than I can come up with. I guess it helps while we wait for some kind of official announcement.
For now, catch Spider-Man: Far from Home which hits theaters July 2.