Personally, I'm fond of the theory that suggests the infinity stones were beings from the time before the big bang and were trapped during the creation of the universe. The eventual destruction of the infinity jewells after the events in Endgame could lead to their freedom and it would be a promising plot element for phase 4, in my opinion.
For example, if the mind stone contained the phoenix force, it's release could lead to the sudden arise of mutants who previously had the mutant gene dormant (which makes sense, since the maximoff twins were enhanced using the mind stone). If this were to happen, then society would focus on the issue of the rising mutants and forget the cosmic menaces for awhile.
On the other hand, the power stone containing Galactus could eventually lead to those major story arcs such as the Galactus trillogy and the Annihilation, provided that the characters involved can be used. Surely, as soon as Galactus is freed, his heralds (Terrax, Stardust, Firelord, Air-walker and the silver Surfer) would come along. And those characters could certainly be used in the Guardians of the Galaxy or Captain Marvel franchises or even in some future major cosmic level Avengers movie.
Inspired byt this theory and since Doctor Strange II is being planned, I think nightmare could be introduced as the being contained in the reality stone. I think it makes sense due to his reality warping related abilities and due to his ocasional red demonic appearance. I must admit I don't know much about this character, however.
Certainly, other characters and plots could be introduced this way.
As for the introduction of the Fantastic Four, I think the best way would be to introduce them separately and inside another franchise (similarly to Spider-man in Civil war) and eventually give them an origin story taking place in the past (like Captain Marvel).