Fan theories and the Marvel Cinematic Universe go together perfectly, and Avengers: Infinity War might be the MCU's biggest fan theory magnet yet. It can be easy to get overwhelmed by the sheer number of theories leading up to the new movie. But don't fret, because in all the noise, there is only one Avengers: Infinity War theory you need to know about before going into the movie. Any MCU fan can tell you that Iron Man and Captain America are the frontrunners to die in the next few Avengers films, but what some less obsessed fans might not know is that it's possible not all deaths will be permanent.
The most important Avengers: Infinity War theory audiences need to be aware of is the time travel theory. The theory is exceptionally complex, with many different variations due to the headache-inducing concept of time travel and alternate timelines. And, as such it's sort of a catchall for theories about what's coming in the MCU. For example, Captain America could die in Infinity War, or he could be sent back to the '40s to live out his life with his first love, Peggy Carter. Injecting time travel into most Infinity War theories changes them slightly, but also works to incorporate a lot of what fans are expecting to see in a brand new unified theory.
To simplify things, the theory is essentially that there are multiple timelines in the film. It's unclear what, exactly, these multiple timelines would be, though one assumes that at least one timeline will find the Avengers failing to protect the planet from Thanos' destruction. What is certain in this theory is that Doctor Strange and the Eye of Agamotto will play a key part. Fans of Doctor Strange will remember that the sorcerer is in possession of the Time Stone and he successfully used it to trap the villain Dromammu in a time loop, keeping him from destroying the Earth.
As seen in the trailers, Doctor Strange meets up with Iron Man and Bruce Banner in the streets of New York City in a scene that appears to be from early in the film. Tony Stark's shocked expression as he looks up at the sky as well as his and Bruce's civilian clothing show that they aren't yet to the point in the film when the Avengers charge into battle. (See photo below.) Assuming Doctor Strange joins forces with Tony Stark early on in Infinity War, it makes sense that he would be able to use the Time Stone to create a larger time loop, one that might be able to reset back to this moment if the Avengers fail to beat Thanos. Even if he doesn't, it's clear that Doctor Strage will be using the Time Stone in Infinity War, as he tells Tony in an Infinity War trailer, "I went forward in time, to see all the possible outcomes."
The trailers for Infinity War have provided many clues to the multiple timelines theory. As ScreenRant points out, different trailers provide two different versions of Spider-Man and Doctor Strange's first meeting. Granted this could just be regular, trailer discrepancies. Blockbuster trailers have been known to use footage from the film that was later left on the cutting room floor (see: Rogue One), and conversely this could just be Marvel trying to throw fans off the scent.
Another small detail from the promos that supports the multiple timeline theory is the Infinity Gauntlet itself. In a commercial advertising Marvel Studio's 10 year anniversary, behind-the-scenes footage shows the Infinity Gauntlet with four Infinity Stones in place, suggesting that over the course of Infinity War, Thanos goes from having two Infinity Stones (as seen in his one-on-one fight with Captain America) to having four.
In the first photo (above), Thanos clearly has the space stone (aka the Tesseract last seen in Thor: Ragnarok), and the power stone (last seen in Guardians of the Galaxy). But in the second photo below, the Infinity Gauntlet is also adorned with what looks to be the reality stone (aka the Aether last seen in Thor: The Dark World) and the mind stone (the one in Vision's forehead) or the Soul Stone (which we have yet to see on screen).
Notice that one of the two stones missing from the Infinity Gauntlet in the second photo is the green time stone. If, in fact, this behind-the-scenes shot is actually a clue as to what happens in Infinity War and not just Marvel showing off a fun new prop, then it seems that Doctor Strange will be holding onto the time stone for as long as possible. And this in turn means that he could very well use it in the final minutes of Infinity War in an attempt to change the outcome of their fight. (This might also confirm rumors that the soul stone is, in fact, in Wakanda, where the Cap vs. Thanos fight takes place.)
Perhaps the biggest piece of evidence to support the Avengers: Infinity War time travel theory is Benedict Cumberbatch's recent admission at being one of the only actors permitted to read the entire Infinity War script. During an appearance on Good Morning America, Cumberbatch confirmed that he'd read the script, while Tom Holland and Tom Hiddleston both insisted that they have not. "I had to sort of understand what the context was," Cumberbatch said. "Strange has this wonderful role in it and I'm trying to kind of hold the multiverse together, so I kind of needed to know what goes on in between."
If Doctor Strange is the one character that really ties the entire movie together, then it definitely stands to reason that the time stone and time travel will feature heavily in Infinity War. How, exactly, the Avengers will use time travel to defeat Thanos remains to be seen. There have also been some theories that suggest that Avengers 4 will be an alternate timeline movie, so whatever you do, get ready to be confused.