Heard a rumor on how the Fantastic Four could be introduced into the MCU and I actually would get on board with that............which means the new/current MCU creative staff will probably not even come close to this and try their damnedest to subvert our expectations when nobody asked for it, all the while making sure the new/current M-She-U boat continues to row full steam ahead....................and with that said, here's the rumor:
The F4 will be introduced in Ant-Man 3 along with Kang, having been held captive in the Quantum Realm since the 1960s. Ant-Man and crew will help free the F4 and introduce them into the modern day MCU.
Then in the stand-alone film, it will go back to the 1960s (which is when the comic first debuted) and serve as the origin for the group and eventually lead to how they were imprisoned in the Quantum Realm.
I think that would be a great way to introduce the F4 into the MCU and explain where the 616 version of the group has been this whole time. (Remember time works differently in the Quantum Realm)