The Negative Zone is an alternate dimension that is very closely related to The Fantastic Four. As the Quantum Realm is to Ant-Man and The Wasp, or as the Dark Dimension is to Doctor Strange, The Negative Zone is to Marvel’s First Family. It just so happens to be the key to bringing the Fantastic Four into the modern world from the 1960s. There is also the fact that the Negative Zone might already be introduced in the MCU.
The Negative Zone functions as a dimension that is outside of all universes. The End of Time has already been introduced as a place outside of timelines, which means that it could, in fact, double as The Negative Zone. However, if the Fantastic Four were to be stuck there, perhaps with Doctor Doom, Doctor Doom may become the new god emperor of Battleworld after deposing Loki.
Doctor Doom deposes Loki, The Fantastic Four Explore the Multiverse
If Doctor Doom somehow finds himself at the End of Time, he could very well take the throne from Loki, given his proficiency in Sorcery. The Latverian ruler could then go on to shape the multiverse in his image, much like Kang the Conqueror, leading to Avengers: Secret Wars.
This is where the Fantastic Four could come into play. After a deposed Loki would find his way back to the TVA, the Fantastic Four could aid him in recruiting heroes across the multiverse by exploring different realities. The Fantastic Four have been known to be explorers in the comics and could take up that role again to aid Loki in assembling a multiversal team to fight God Emperor Doom.
Doctor Doom could then serve as the main antagonist of Avengers: Secret Wars, which is allegedly set to be a soft reboot for the MCU, setting a slew of things in motion to bring the X-Men and the Fantastic Four into the mainline Marvel Cinematic Universe. Doom would be the perfect replacement for Jonathan Majors‘ Kang, with a similar move set that the character has.
Galactus could be the Hail Mary the Avengers use to defeat Doom
Galactus is one of the most powerful characters in the Marvel Universe, so much so that more often than not, the heroes have to come up with new and inventive ways to defeat the character. Even then, Galactus is often described as a force of nature, something that just keeps on coming with no end in sight. This is why, one of the many solutions that Reed Richards comes up with is to send Galactus into The Negative Zone.
If The Negative Zone and the End of Time are the same places, then it is possible that Galactus could be brought in to fight whatever antagonist the Avengers find themselves up against, as a means of occupying the cosmic entity, while also attesting his power, along with whoever they are fighting. The Avengers could use Galactus as a distraction for God Emperor Doom, while Loki and the TVA either seal the End of Time or make it impossible for its inhabitants to leave, closing Battleworld to the wider multiverse.