Ever since the Tesseract was introduced in the Thor post-credits scene and Thanos cameoed at the end of The Avengers, the Marvel Cinematic Universe has been building up to its own version of the Infinity Gauntlet storyline, namely with the Mad Titan slowly obtaining the six Infinity Stones. That will culminate in 2018 with Avengers: Infinity War, and elements of the storyline will presumably be touched on in Avengers 4. Once Phase Four hits, though, the door is wide open for a new storyline or two to be used across multiple movies.
While it remains to be seen whether Marvel's Phase Four would use a story that reaches as wide as the Thanos/Infinity Stones arc, we've collected some of the classic storylines/events from the Marvel comics that would fit well on the big screen. Full disclosure, we would love to see the MCU heroes battle Galactus, the world eater, but until some kind of deal can be worked out with 20th Century Fox over the Fantastic Four rights, that will remain a dream. Fortunately, there are plenty of other options available.
There are three primary Marvel events to use the Secret Wars name: the one from 1984-1985, the sequel that followed soon after, and the one from 2015 which changed the Marvel universe as we know it, literally! That 2015 event is the one we specifically have our eye on for cinematic adaptation. Think about it, after Thanos has put the entire universe at risk, how do you raise the stakes from there? Introduce other alternate universes, a concept which has already been established in Doctor Strange. We don't expect a 100% faithful adaptation of the Secret Wars miniseries (in fact, that's impossible since Marvel can't use Doctor Doom), but it would be interesting to see the main MCU suddenly find itself opened to other realities that are filled with alternate versions of characters we know and love. That said, should this storyline last across one Phase or several, having Battleworld be the endgame would make for a great conclusion.
The Korvac Saga
In the comics, Michael Korvac was a computer technician from an alternate Earth who traveled to "our" universe and became ridiculously powerful thanks to a chance encounter with Galactus. Upon him being imbued with the Power Cosmic, the Avengers and the original Guardians of the Galaxy had to team up to stop the villain. Most of them actually died in the process, and it was only due to Korvac actually committing suicide and bringing the fallen heroes back to life before he died that life continued as normal afterwards. While Korvac coming from another Earth might not work for the MCU, using a normal human who is given extraordinary powers from an alien source works, with him (or her) becoming so mighty and god-like that the Avengers and Guardians once again have to join forces to defeat him. This is one of those storylines that could only work for one movie, maybe two max, but it's still ripe to bring into live action.
We've already seen Norman Osborn as the Green Goblin in 2002's Spider-Man, and while some may be eager to see a new version of him in one of the Spider-Man: Homecoming sequels, it would be more captivating if he started out as the suit-wearing, corporate mogul, politically scheming evildoer we see in the Dark Reign and Siege events. However, instead of the Skrulls' invasion of Earth being the cause of his rise to power, show Norman slowly accumulating resources and support in a few post-credits scenes. Then in an Avengers flick (or something of equal importance), he can launch his attack on the MCU heroes, whether it's with a Dark Avengers team, controlling Sentry or striking some kind of deal with Loki to destroy Asgard. By the end, Norman's defeat would drive him to insanity, paving the way for him to become Green Goblin in a later Spider-Man movie. Yes, it's a backwards approach to his super villain guise, but narratively speaking, it could make sense.
World War Hulk
Many Marvel fans have been hoping for a Planet Hulk movie, but the closest they'll come to that are with the elements from that story being used in Thor: Ragnarok next year. World War Hulk, on the other hand, could still work. Maybe by the end of Avengers 4, Hulk is launched back into space, and when he returns to Earth, he's brought an army with him to exact revenge. Whether the Hulk is acting this way naturally or if he's been corrupted somehow, his power level is so high that it will take everything the remaining Marvel heroes have to stop him and his followers. As for the issue of Marvel being unable to release any more Hulk movies, just have it be an Avengers movie with the subtitle World War Hulk. With all the protagonists showing up to battle him, it can't exactly be labeled as a "solo Hulk adventure."
Right off the bat, let's clarify that the MCU would need to use the basic framework of the Kree-Skrull War rather than directly tackle it. Just like with Galactus, the Skrulls are part of the Fantastic Four "family," meaning that Marvel can't use them. Fortunately, there other alien species that can used as substitutes, ranging from the Chitauri to the Badoon. Getting back on track, if Marvel is looking to keep the Earth-bound heroes still involved in cosmic affairs, throw them into a conflict between two extraterrestrial factions. This could easily be teased/last across multiple movies, not to mention introduce older elements from the Marvel comics universe, like Mar-vell or Vision and Scarlet Witch's romance. After the battle with Thanos, one would imagine that the barriers between what happens on Earth and out in the cosmos will be weakened, so use Phase Four as a way to connect the Earth heroes to intergalactic current events.
Contest Of Champions
Most of you know Contest of Champions as a mobile game, but long before that, it was a limited series where the Marvel heroes were forced to battle each other in a twisted game between Grandmaster and Death. Jeff Goldblum is playing Grandmaster in Thor: Ragnarok next year, and hopefully that isn't just a one-time-only appearance. As for death, rumor has it that Cate Blanchett's Hela will fill the Death role in the MCU, meaning that she presumably can't be permanently eliminated. These are two entities that can surpass Thanos (without the Infinity Stones, that is) in terms of power/prestige, so why not have them pull the strings in Phase Four? The heroes would realized they're being used as pawns, and the storyline would conclude in the main Contest of Champions battle. Since the original reason for the game was to decide the fate of The Collector, this would also be a way to highlight Benicio del Toro's iteration of the character outside of the Guardians of the Galaxy movies.