Avengers 4 is going to feature some form of time travel - but why, and how will it work? With Avengers: Infinity War now out for more than a month, Marvel fans' focus has almost fully shifted to next year's Part 2.
Marvel Studios has been very cagey about what people can expect from the MCU Phase 3 capper, but between leaked set photos and pieces of information revealed regarding the movie, as well as other forthcoming Marvel releases, it's safe to say that time travel will be a major factor in Earth's Mightiest Heroes' final bout with Thanos.
It's currently unclear how time warping would be possible in Avengers 4, especially since the MCU hasn't really utilized the concept, bar some minor dabblings by Doctor Strange. However, there are actually a few ways on how the MCU can go about it - depending on why they're doing it in the first place.
THE QUANTUM REALM
Initially introduced in 2015's Ant-Man, the Quantum Realm - "a reality where all concepts of time and space become irrelevant as you shrink for all eternity," as explained by Hank Pym - is one of the most-discussed ways that the remaining MCU heroes can go back in time. It's already been established that the Quantum Realm will play a significant role in the MCU moving forward with the alternate dimension being a pivotal part of Ant-Man & The Wasp and Captain Marvel. As they're both releasing before Avengers 4, it's been theorized they're actually roundabout setup for the team-up.
Ant-Man & The Wasp is expected to further explore the Quantum Realm via Pym Particles (it could also be entered with the use of Doctor Strange's Sling Ring) - the same particles that allow the titular heroes to change in size. Trailers for the film have teased a vessel that can be used to travel to the Quantum Realm without risking the possibility of being trapped in the multiverse by going subatomic. The aforementioned craft will be used by the heroes attempt to save the original Wasp, Janet Van Dyne, who has been trapped in the Microverse for decades. Captain Marvel, meanwhile, has been theorized to use the Quantum Realm as a way to explain why Carol Danvers has been absent for the past 20 years.
Given two time displacement uses are coming up, it would extend that the surviving Avengers members like Tony Stark and Steve Rogers could enter the Quantum Realm and use it to go to different points in time - possibly with the help of new sign-up Ant-Man.
TONY STARK DEVELOPS TIME TRAVEL
It's not a secret that Tony Stark will play a pivotal part in Avengers 4 considering how connected his personal narrative arc is with Thanos' arrival. Doctor Strange's decision to give up the Time Stone to save Stark's life after the Sorcerer Supreme peeked into the future is further indication that the MCU's founding hero will be significant in ultimately defeating Thanos. While there are several ways for him to contribute, developing a way to travel back in time is undoubtedly within the realm of possibility considering his scientific genius.
The Russo brothers have previously shared that Stark's Binary Augmented Retro Framing aka. B.A.R.F - a technology that hijacks the hippocampus to clear traumatic memories, introduced in Captain America: Civil War - will play a significant part in Avengers 4. At that time, Stark has made it clear that the tech could not alter reality but simply help people deal with regret and depression. However, that doesn't mean that he won't continue to tinker with his creation if he learns that it might be the answer to his Thanos woes. During his M.I.T. speech, Stark has also specifically ended his presentation by saying "Reframe the future" which could very well be a foreshadowing of how he's going to do just that with the use of an improved B.A.R.F.
The third option would be the obvious, albeit most difficult route: using Time Stone. Considering that Thanos has the stone and Doctor Strange is currently a pile of dust, there's some barriers, although given the Mad Titan believes that he has now accomplished his life-long mission, he is open to attack from the Avengers.
The question becomes who attains it. Avengers: Infinity War has established that Nebula almost succeeded in assassinating Thanos while he was aboard the Sanctuary, and with her adoptive father no longer operating at a high alert level - not to mention the loss of the Black Order - sneaking in to snatch one of the Infinity Gems will be easier.
This also gives Nebula a strong character arc in Avengers 4. In the comics, she has a major role in defeating her adoptive father, but Infinity War sidelined that, giving the spotlight to Gamora instead. However, with Gamora supposedly gone, this paves the way for Nebula to fulfil her print narrative.
TIME TRAVEL CAN SAVE THE DEAD HEROES
The obvious reason for time travel is to find a way to resurrect the heroes snuffed out from existence by the end of Avengers: Infinity War. After all, most of the characters that turned into dust are confirmed for their own respective sequels, ensuring that they'll be revived one way or another. For instance, Chris Pratt, Pom Klementieff, and Dave Bautista have all confirmed that they're returning to reprise their respective roles - Star-Lord/Peter Quill, Mantis, and Drax the Destroyer - in next year's untitled Avengers, as well as Guardians of the Galaxy Vol. 3. Tom Holland, on the other hand, is all geared up to start filming Spider-Man: Homecoming 2, while it's commercial suicide to not see Chadwick Boseman as T'Challa again considering how successful Black Panther was.
This is surely going to be a key drive in Avengers 4, and if they really are dead, time travel could be the only way to save them.
THE AVENGERS TRY TO GET THE STONES BEFORE THANOS
If the plan is bigger, however, then time travel has a much more intriguing role: what if the Avengers are trying to redo the MCU and get to the Stones before Thanos?
Avengers: Infinity War made sure to show an extra gauntlet left lying in the forge on Nidavellir, and while its existence wasn't explicitly discussed during Thor and King Eitri'sconversation, it could be used for a similar purpose: it's an initial sample of what the Mad Titan was using, enabling it to also hold all six Infinity Stones. If the Avengers want to use the Stones against Thanos, that provides an established way for them to do so.
Of course, Thanos already has all six Stones, making him a formidable task. Going backwards before he was at full power may actually be an easier solution.
THE AVENGERS BUILD A FLAW IN THANOS' VICTORY
Leaked set photos for Avengers 4 reveal that the remaining heroes travel back in time to revisit the Battle of New York, the event where Thanos' presence in the MCU was really felt for the first time. While there's lots of reasons they could be here - it was when the Avengers first assembled and involved two Infinity Stones - it would fit as a specific point in history that needs to be re-examined.
Based on the images, it appears that Tony and Thor will attempt to meddle with Loki's arrest at the end of the movie, with the former disguising himself as one of the S.H.I.E.L.D. agents who handled the God of Mischief's capture. Given Loki's importance after these events, there's possibly some meddling with the timeline, rather than simple stealing of Stones going on.
Whatever the reasoning or methodology, that Avengers 4 will be dabbling with time travel seems pretty certain at this point. Avengers: Infinity War set up Thanos as an unstoppable threat, and so something so desperate to beat him on makes sense for the sequel.