We'll have to see where they go from here with the next Big baddie who will get several post credit Scenes build up.
A common guess I have seen was Galactus.
Well, at least they can't do a worse job than Fox did with Galactus!
In regard to the acquisition of Fox, I do see Fantastic Four being far more likely to see a faster introduction into the MCU than X-Men. The Fantastic Four have only had three movies so far, and the last one was so maligned that I think most would welcome a better showing. On the flip side, X-Men has had some more popular movies in the recent years, but it's arguable that they've been getting a bit worse as time goes on. X-Men also has two movies coming out this year (
Dark Phoenix and
New Mutants). Also, introducing mutants brings up interesting questions about characters like Scarlet Witch and Quicksilver (RIP) given that they're mutants and Magneto's children in the comics, but are just "special" in the MCU (experimented on with the Mind Stone).
I think it also helps that Fantastic Four only has four main characters to worry about. On the other side, X-Men has a rather large cast, but that could also be an interesting advantage. For example, there has been a decent amount of fan demand for a good Gambit movie, and given that he was only featured in X-Men Origins: Wolverine, doing something like a solo Gambit film that starts to introduce mutants into the MCU might not be a bad idea.
Gambit was actually supposed to come out in 2020, but given all the delays it has seen, that was still a bit tenuous at best. Apparently, Marvel is looking into the movie, which might be a good sign.
Outside of existing characters, some of the rumored fresh faces with be
Shang-chi and
The Eternals.