Going back to ways to potentially link the Fantastic Four to this movie, the Celestials could be a great way to do it. The Celestials are essentially a group of godlike cosmic entities who have existed in the Marvel universe since its birth. They are each tasked with a different responsibility, such as judging whether the civilisation of a planet will live or die, surveying and mapping the geography of planets and, in one case, maintaining and monitoring Apocalypse himself. The reason for this is because Apocalypse's own powers are augmented through the use of Celestial technology. The mutant acquired said technology when the cosmic beings loaned him it in exchange for "great sufferings" later. After their potential introduction in X-Men: Apocalypse, the Celestials could become characters in the Fantastic Four series of movies. They have appeared in numerous Fantastic Four comics in the past, interacting with both the titular superhero team and Doctor Doom.
