(Warning: This post is basically made out of "Avengers: Infinity War" spoilers. You might want to wait until you've seen the movie before reading on. Also, if you like this story, you'll also like this list of the 6 Most Plausible "Avengers: Infinity War" Fan Theories.)
When "Avengers: Infinity War" finally hit theaters, it brought together stories from across the Marvel Cinematic Universe. It also ended with a huge cliffhanger and some major repercussions for Marvel's movies going forward.
There's another year before "Avengers 4" shows up to explain what's happened and resolve the story, and already, fans are putting all their mental powers and Marvel comics knowledge into guessing at what could happen with Thanos (Josh Brolin), the Marvel heroes and the Infinity Stones.
(Seriously, we're about to talk about the ending. Last warning!)
Fans mostly are trying to figure out how the heroes might defeat Thanos and undo the end of"Infinity War." That ending, of course, saw Thanos getting hold of all six Infinity Stones and using them and his Infinity Gauntlet to cause half the people in the universe to cease to exist. In the comics, the heroes manage to undo that action, and fans theorize that the MCU won't allow half its heroes to remain as piles of dust for long.
"Infinity Wars" fan theories range from relatively plausible to fairly weird, drawing on ideas about multiple universes, cosmic entities and unseen characters. Here are some of the wildest Avengers fan theories out there right now.
Death helps out
"Infinity War" adapts a lot of the stuff about Thanos that makes it into the comics, but it skips over one of his key motivations: He's in love with Death. That's Mistress Death, the metaphysical manifestation of the concept, who Thanos repeatedly tries to impress in the comics in hopes of wooing her. Obviously, "Infinity War" skips that part of his whole "murder half the universe" plan. As directors the Russo brothers and writers Stephen McFeely and Christopher Markus have put it, introducing a new character in Mistress Death (along with Thanos), plus the idea of living embodiments of metaphysical concepts, was a little too much heavy lifting for the movie.
But since "Infinity War" gave Thanos a lot of the focus, there could be room to add another character in the sequel. We also know that the Marvel Cinematic Universe has been willing to get a little weird with its cosmic goings-on -- the Infinity Gauntlet itself is a big wish-fulfilling device, and finding Red Skull (Ross Marquand) as the guardian of the Soul Stone suggests some weird, unknowable stuff. In the comics, Mistress Death stood against Thanos after he wiped out half the universe, along with a ton of other cosmic heavy hitters, such as the Elders of the Universe -- including the Grandmaster (Jeff Goldblum) and the Collector (Benicio del Toro). Potentially, she could pop up to help out here.
The Quantum Realm leads to the Multiverse
The positioning of "Ant-Man and the Wasp" on the release calendar between "Infinity War" and "Avengers 4," as well as Marvel's teasing that the "Ant-Man" sequel will reveal what Scott Lang (Paul Rudd) and Hope van Dyne (Evangeline Lilly) were doing during "Infinity War," suggests the movie will influence the conclusion of the Thanos story. (For a whole lot more on how "Ant-Man and the Wasp" could factor into "Avengers 4," read this.) The assumption so far is that the thing that will matter to the Thanos situation is contained in, or has to do with, the Quantum Realm, the super-small, subatomic universe reachable only by shrinking. But nobody knows what might be found there, or what role the Quantum Realm could play.
One fan theory suggests the Quantum Realm might be a bridge to the Multiverse. In the Marvel comics, there isn't just one universe in which the action takes place, but rather several parallel universes that are all different. In fact, the main comics continuity takes place in one universe (known as Earth-616), while the MCU composes another (Earth-199999). If the Quantum Realm leads to other universes, that could give the heroes a chance to visit places to gather their Infinity Stones from those universes, allowing them to use Thanos' power against him. It might be a big leap, but travel between the universes has occurred before in the comics, and would expand the MCU in some useful ways.
Riffing on the Multiverse idea is that Thanos' universe-massacring "snap" didn't actually kill anybody. Instead, the Infinity Stones whisked all those folks off to another universe, where they're still alive -- and could potentially make it home to their own universe.
Nebula could wind up the villain of "Avengers 4"
Thanos' daughter Nebula plays a major role in the "Infinity Gauntlet" comics storyline from which "Infinity War" is largely adapted. In his efforts to impress Mistress Death, Thanos didn't go straight to gathering the Infinity Stones and killing half the universe. First, he started by torturing Nebula (his granddaughter in the comics) in incredibly horrific ways, burning her and leaving her so broken that she was basically borderline mad.
In the comics, it's Nebula who ultimately undoes Thanos' destruction of the universe, after he kills pretty much all the heroes and wrecks most of the super-powerful cosmic beings out there. When Thanos ascends to near-godhood thanks to the Infinity Stones, he takes on a cosmic, ethereal form, with his spirit leaving his body. Nebula grabs the gauntlet off his unguarded body and uses it to heal herself, and then she controls the universe. But she's tricked into restoring the universe to its state 24 hours earlier, saving everybody. (Here's everything you need to know about how the story went in the comics.)
Fans speculate Nebula (Karen Gillan) survived Thanos' culling so "Avengers 4" could use her in a similar way. If she gets the Infinity Gauntlet off Thanos, she'll surely get her revenge on her Mad Titan father -- but she's also not the most stable person, and could easily become a problem to be dealt with in "Avengers 4." Who knows what a Nebula-shaped universe could look like.
Thanos didn't make it
At the end of "Infinity War," Thor (Chris Hemsworth) shows up just as Thanos gets hold of the Mind Stone with his new beefed-up Stormbreaker Axe and buries it in the Mad Titan's chest, doing serious damage. Thanos survives long enough to snap his fingers and make his wish to wipe out half the universe come true. Then he bails, stepping backward through a Space Stone portal to make his escape. Right before that, though, he finds himself in an orange-tinted place with young Gamora (Zoe Saldana) (seemingly the Soul World, a place that exists inside the stone in the comics). In the closing moments of the movie, an injured but alive Thanos settles down to watch the sunrise over "a grateful" universe, but what if he didn't actually survive the battle?
Thanos appeared to be very hurt by the use of the Infinity Gauntlet, and the events of "Infinity War" are heavily focused on the Soul Stone and how it works. A dead Thanos would fit in a continued focus on the Soul Stone. But his death would also create new issues -- specifically, the lack of anyone powerful enough to wield the Infinity Gauntlet to undo what Thanos had done. It might even lead to a new requirement of teaming up with Thanos in order to revive him, if his soul is trapped in the Soul Stone as "Infinity War" might be implying, so that he can fix the damage he's done.
Skrulls have invaded Earth, and they have a part to play
Part of the story we know will be told in "Captain Marvel" is that of an alien race called the Skrulls waging a war against another group of aliens, the Kree. We saw the Kree and their interstellar empire briefly in "Guardians of the Galaxy," but while Skrulls are a major part of the Marvel comics universe, they have yet to show up in any other movies. The Skrulls have shapeshifting capabilities, meaning they can mimic human form and blend in on Earth, and some can even take on the powers of the people they mimic, creating superhero versions of Skrulls. (We've got a deeper dive on the Skrulls and "Captain America" right here, and here's a big rundown of how "Captain Marvel" might factor into the Thanos conflict.)
The Skrulls show up a fair amount in the Marvel comics, and while "Captain Marvel" might resolve the Kree-Skrull war on Earth during the 1990s, it seems doubtful that would be the end of the Skrulls altogether. In fact, there's some speculation that Loki's invasion of Earth with the Chitauri aliens in "The Avengers" could have covered for a more clandestine Skrull invasion. If there are still Skrulls on Earth, they could potentially pop up to be a part of the Thanos conflict once they're introduced in "Captain Marvel," serving as either villains to be dealt with or, possibly, allies against the Mad Titan. After all, it's the Skrulls' universe, too.
There's a seventh Infinity Stone
Another possibility fans have brought up is that there's another Infinity Stone out there, one which houses the overall consciousness of all seven stones. We've seen in the MCU movies the stones have some kind of sentience or life -- in "Infinity War," the Mind Stone tries to warn Vision (Paul Bettany) about Thanos, for instance. In the "Ultraverse" storyline of the Marvel Comics, the Infinity Stones weren't just created at the Big Bang: They were once part of an all-powerful being called Nemesis. Lonely, she broke her power apart into the stones and scattered herself across the universe, with the seventh stone, the Ego Stone, housing Nemesis' consciousness. So there could be another stone out there for the heroes to find and use. It could remove Thanos' capabilities by reviving Nemesis.
There's also another precedent for a seventh Infinity Stone. One short-lived element of the comics stories has Thanos using the stones to create a seventh, synthetic stone, which was able to hold the powers of the other six stones. He planned to extinguish all the stars in the universe with its power. We know Shuri (Letitia Wright) scanned the Mind Stone to save Vision (who might still be alive). Between her information and what Tony Stark (Robert Downey Jr.) and Bruce Banner (Mark Ruffalo) discovered in "Avengers: Age of Ultron," making a new stone could be possible.
There are souls in the Soul Stone, and they'll fight Thanos
The Soul Stone in the comics is the home of the Soul World, and souls of people killed by the stone wind up inside it, allowing its user to access them and their powers. A prevailing fan theory about "Infinity War" is that everyone who ceased to exist thanks to Thanos' snap didn't die, but that instead their souls are held in the Soul Stone. The movie already suggested Gamora's soul was held inside the stone, so it's not a huge stretch.
The fan theory about the Soul Stone suggests that the heroes inside the stone could work together to find a way to fight Thanos from within his own seat of power. There's some precedent for that in the comics, too. In "Infinity Gauntlet," a character we haven't seen in the MCU yet, Adam Warlock, basically hung out in the Soul Stone for most of the fighting against Thanos, biding his time. When Nebula got the Gauntlet away from Thanos, Warlock used his connection with the stone to create pain in the glove, causing Nebula to take it off so he could snag it. We don't know how Doctor Strange or any other heroes potentially trapped in the Soul Stone might oppose Thanos, but their new form could give them an edge the Mad Titan doesn't expect.
There are other Infinity Stone seekers, and they'll matter
When Thanos journeyed with Gamora to Vormir to find the Soul Stone, they ran into its keeper, the Red Skull. The villain of "Captain America: The First Avenger" had some cryptic things to say, but basically, he talks about how the Space Stone rejected his attempts to possess it, and he's now bound to the Soul Stone, destined to lead people to it but unable to possess it himself. The Red Skull also mentions that Thanos is not the first person to try to take the stone -- which raises some questions about what happened to those other folks, and if they actually failed to get it. (Here's some more on Red Skull's possible role in "Avengers 4.")
The theory here is that other powerful beings who attempted to possess the stone and are now, potentially, bound to it like the Red Skull, and could be arrayed against Thanos, maybe even by the stones themselves. The idea of other people who tried to possess the Soul Stone leaves open a big question of who they are and what their deal might be.