The Soul Stone Theory is, technically, not really a theory anymore. Co-director Joe Russo confirmed that the final Thanos and Gamora scene in Infinity War did take place inside the Soul Stone (or in the Soul World), which would suggest that all the other Avengers who were killed by the Soul Stone in the end are also not completely dead, but somewhere in the Soul Stone. Thus far, there hasn't really been one cohesive theory as to how Peter Parker, T'Challa, and the others will actually escape the Soul World and continue to fight Thanos, though as the most mystically powerful of the group, Scarlet Witch and Doctor Strange are pretty good bets.

One theory about how the dusted Avengers will return is that the original Avengers will have to sacrifice themselves to save the new class of heroes. While it seems somewhat unlikely that Marvel will kill off Hulk, Captain America, Iron Man, Black Widow, Hawkeye, and Thor (especially Black Widow, who is rumored to have a solo movie in development), this theory could definitely hold true for Captain America and Iron Man, who both have histories of self-sacrifice.

The Quantum Realm was hinted at in Ant-Man and early trailers seem to have confirmed its reappearance in Ant-Man and the Wasp. This theory isn't fully formed yet, in that fans haven't quite figured out how the Quantum Realm will factor in the defeat of Thanos in Avengers 4, but given the timing of the new Ant-Man film's release and all the hype surrounding the multiverse, it's a pretty safe bet that the Quantum Realm will have some kind of role in Avengers 4.

Similar to the Quantum Realm theory, the Doctor Strange multiverse theory gives a key role to the multiverse, suggesting that Doctor Strange may have hidden the real time stone somewhere within the multiverse, giving another universe's stone to Thanos. This means that there are other stones out there somewhere in the multiverse, which could theoretically be used to stop Thanos and reverse the damage he has caused.

Jumping off the multiverse theory, some people believe that the Avengers will go back in time to the Battle of New York. Perhaps to change history and keep Thanos from ever laying his hand on the tesseract, or, as another theory suggests, to create a new army of Avengers picked up throughout time to fight Thanos.

One interesting new theory suggests that Thanos' snap and use of the Infinity Gauntlet may have broken time entirely. As explored by ScreenRant, this theory is based on the Doctor Strange warning that time is sacred and not to be messed with. Any alterations using the time stone are small breaks in time, and Thanos might have bitten off more than he can chew by using it.

In the comics, Nebula defeats Thanos by taking hold of the Infinity Gauntlet and using it to undo his evil actions. It's possible that she could play a similar role in Avengers 4, especially because she did not dissolve into dust, making her one of the few allies the Avengers have. In the comics, the power of the Gauntlet consumed her and turned her into a villain, which would also be a great twist to the film.

There are a lot of different ways that Captain Marvel could contribute to Avengers 4. As a character historically familiar with the multiverse, it's possible that her role could be tied to the Quantum Realm or Multiverse theories above. No doubt Captain Marvel theories for Avengers 4 will become more clear once the Captain Marvel movie hits theaters. Until then, it's safe to assume that whatever role she does play in Avengers 4, it's going to be big.

Parallel to the Doctor Strange multiverse theory is the theory of a bonus Infinity Stone. Known as the Ego stone in the comics (there is also a bonus Death stone in other comics), this bonus Infinity Stone could be the secret weapon needed to defeat Thanos. As described on Marvel's official website, the Ego Stone is hidden in the Ultraverse, a realm unknown to the Avengers. It's powers are beyond those of the Infinity Stones we already know, and the Avengers could theoretically use it to defeat Thanos.

There are some that believe that everyone who turned to dust at the end of Infinity War is really dead. Unlikely as it may be, it is still, technically, an Avengers 4 theory, but the suggestion alone that we've seen the last of Spider-Man and Black Panther is enough to make this theory the least likely in the whole MCU.

OK, this isn't really a theory, but it's quite possible that none of the theories above will actually happen in Avengers 4, and that the next film will be just as depressing as Infinity War's tragic ending. As difficult as it is to imagine a villain capable of bringing down the Avengers for good, Thanos might beat the odds and surprise us all in Avengers 4.

As fans struggle to recover from Avengers: Infinity War, the phrase "Avengers Assemble" won't have quite the same ring to it in Avengers 4 as it once did in The Avengers. And if some of these theories are correct, it never will again.