I think Fantastic Four will come first. I think they've already been prepping the MCU for the Fantastic Four.
1) The X-Men movies have been solid. Not great and not horrible. They still have Dark Phoenix coming out this year and I think that will delay them a few years.
2) Fantastic Four has had 3 tries and none of them really landed. It's much easier to let go of those movies and forget them than the X-Men movies in the short term immediate future.
3) I'm 99% sure the new owner of Avengers Tower that we didn't conveniently meet in Homecoming is Reed Richards. That will setup Avengers Tower to become the Baxter Building.
4) A lot of purists won't like this, but got streamline purposes I believe the Negative Zone and the Quantum Realm are going to be the same thing in the MCU.
5) It would be easy to retcon Reed and his family into the Ant-Man lore with Hank Pym. They discovered the Quantum Realm together back in the 60s. That should be where the first Fantastic Four movie takes place. End result of the movie is them getting stuck in the quantum realm like Janet and getting sucked into a time vortex. They would finally escape in present time, having not aged at all.
6) Post credit Endgame is Happy handing the keys to the new owner of Avengers Tower and asking his name, to which he replies Reed. End of credit scene.
