Marvel Studios' Phase Four announcement didn't consist include a big crossover event, or even an Avengers film, for that matter, underscoring that, post-Endgame, the Marvel Cinematic Universe will indeed be vastly different.
However, there are elements in place that indicate a possible direction for the franchise to take with its next status quo-altering event. From Phase Four to Phase Five, it appears as if the MCU is building toward a cosmic event. Given the evidence, it is becoming increasingly probable it will be based on Marvel Comics' Annihilation.
What is Annihilation?
Debuting in 2005, Annihilation was a crossover in which Marvel's cosmic characters battles the forces of Annihilus, a classic Fantastic Four villain, who unleashes his army of soldiers from the Negative Zone upon the universe. Xandar and the Nova Corps initially confront the threat, but soon the Kree and Skrulls are forced to deal with the coming tide.
But events are complicated when Annihilus captures and dominates Galactus himself, harnessing the power cosmic for his own. The battle escalates, resulting in an alliance of the Guardians of the Galaxy, Silver Surfer, Nova, Super-Skrull and others.
The 2007 sequel, Annihilation: Conquest, deals with the aftermath, in which Ultron leads a cosmic armada to spread devastation throughout the universe. Marvel Comics announced a follow-up to Annihilation at Comic-Con International, which could be only a coincidence. Ahem.
The Divide Between Cosmic & Earthly Stories
There's increasing evidence that Annihilation (or some variety of it) is coming to the MCU. In an interview following Marvel's Hall H presentation, studio President Kevin Feige acknowledged that he already knows the Phase Five line-up and crossover event. That would, of course, suggest the Phase Four slate laying the foundation for that -- whatever it is.
There also appears to be an increased divide between the Earth-based and cosmic stories. In Phase Four, there will be more grounded and terrestrial The Falcon and The Winter Soldier, Black Widow, Shang-Chi and the Legend of the Ten Rings and Hawkeye, while the cosmic stuff only appears to be spiraling into something larger.
Eternals, Doctor Strange in the Multiverse of Madness, Loki and Thor: Love and Thunder, with Guardians of the Galaxy Vol. 3 and Captain Marvel 2 in development. Add to this the increase of Skrull activity seen in the post-credits scene of Spider-Man: Far From Home, and it's clear something cosmic is coming.
While Doctor Strange in the Multiverse of Madness might seem like an odd choice to include with the other cosmic films, remember that the Negative Zone is another dimension -- one that causes utter misery and suffering to all who enter it.
Furthermore, there has been much discussion about introducing the hero Nova, a member of the Nova Corps already seen in Guardians of the Galaxy. Nova plays a crucial role in Annihilation.
The cosmic universe is expanding, but there's one crucial element still missing.
The introduction of the Fantastic Four into the MCU will change everything. It seems only logical that Marvel Studios would make a Fantastic Four film in some way crucial to the evolution of the cinematic universe.
So why not introduce Annihilus and the Negative Zone as the villains? Or Galactus and the Silver Surfer? All of those are key characters who play crucial roles in the Annihilation storyline, and all of them could appear in the Fantastic Four film.
Of course, in the MCU, the Celestials can, for the time being, take Galactus' role in the story if Marvel doesn't want to incorporate the Devourer of Worlds just yet. The Fantastic Four would be critical in establishing Annihilus as a threat.
Moving Away From the Avengers
One interesting subject worthy of discussion here is this: Would an Annihilation movie be an Avengers film?
Following Avengers: Endgame, there seems to be little to tie the greater cosmic elements to the Earth elements. Yes, New Asgard is on Earth and its Queen Valkyrie will be on a love-quest in Thor: Love and Thunder. But beyond that, there is no Thanos who can pose a threat to both Earth and greater universe. At least, for now.
Annihilation can do one of two things: It can unify the Marvel multiverse once more, or further divide it into a cosmic- and terrestrial-based world. "I love that things build up," Feige said in that Comic-Con interview, "they build to a crescendo, they all get together, they separate, they change."
Endgame was that crescendo. Perhaps Annihilation can be where the divided pieces change and grow on their own, establishing the cosmic universe as its own thing with its own concerns. Or perhaps it can unite the MCU once more.
Either way, it seems inevitable that the Negative Zone will be launching its invasion of Earth sooner rather than later.