Warning: SPOILERS for Avengers: Infinity War are ahead!
The intrepid heroes of the Marvel Cinematic Universe have been through a lot over the last decade, but it's safe to say that Avengers: Infinity War marked their biggest defeat yet. Despite their best efforts, the Avengers, Guardians of the Galaxy and the other protagonists involved were unable to stop Thanos from obtaining the six Infinity Stones, and with the snap of his fingers, the Mad Titan wiped out half of life in the universe. It was definitely the most depressing MCU movie ending yet, and we still have a little less than a year to go until we find out how this arc wraps up in Avengers 4. Officially, Marvel Studios is keeping specific details concerning Avengers 4 close to the proverbial chest. Unofficially, certain details have come to light hinting that Avengers 4, as impossible as it may seem, will be even crazier than Infinity War.
Since late last year, numerous set photos have popped up showing that Avengers 4 will be re-visiting key moments in MCU history. Arguably the most notable of which is The Battle of New York from 2012's The Avengers, which saw Earth's Mightiest Heroes officially forming as a team for the first time. It will be a while until Marvel Studios sheds light on the context of these scenes, but fortunately there's been no shortage of speculation on how these moments are even possible.
This all started back in October, when Hayley Atwell, best known for her portrayal of Peggy Carter, mentioned on social media that she was in Georgia, where both Avengers: Infinity War and Avengers 4 were shooting for all of last year. Atwell's more recent comments indicate that another Peggy appearance might not be as set in stone as some fans assume, but then set photos uploaded in November showed 1940s-era cars, indicating that we would indeed be going back to the Captain America: The First Avenger era. Granted, working off that alone, it's possible that Avengers 4 might simply be flashing back to the years that Steve Rogers originally hailed from. Then things got more interesting.
Last December, stunt performer Heidi Moneymaker (best last name ever) posted pictures on Instagram showing Scarlett Johansson wearing the same clothes and rocking the same hairstyle that she had at the beginning of Captain America: Civil War, when Black Widow and some of the Avengers were in Lagos, Nigeria to apprehend Brock Rumlow, a.k.a. Crossbones. This followed a report months earlier that Frank Grillo had been seen on the Avengers 4 set. So rather than Crossbones being miraculously resurrected, we would be seeing him as he was in Civil War. Finally, we come to the Battle of New York pictures, which originally showed up in January and have been making the rounds again in recent weeks. Some of them shows Chris Evans wearing the same Captain America uniform he had on in The Avengers. Paul Rudd's Ant-Man and Robert Downey Jr's Tony Stark were also in the scene, although Tony had blonde-tinted hair and was wearing a S.H.I.E.L.D. jumpsuit. Oh, and there were other pictures showing Mark Ruffalo wearing a motion capture suit and carrying a giant bindle, which is... just weird. In any case, along with Avengers 4's present day events, the movie will also be messing around with the past, which leads us to the first theory about how this is possible.
While Agents of S.H.I.E.L.D. introduced time travel at the start of Season 5, the movies have yet to incorporate it. Yes, Doctor Strange is capable of temporal manipulation with the Eye of Agamotto, a.k.a. the Time Stone, but we still haven't seen anyone on the cinematic side of things actually been dropped off somewhere in the past or future. But perhaps Avengers 4 will involve some, if not all, other surviving heroes somehow going back in time to change certain events that will ultimately lead to Thanos not succeeding in Avengers: Infinity War. Remember, Thanos carried out genocide on a universal scale, so you can imagine that the heroes who are still alive would do anything to undo this devastation. However, Thanos still possessed the six Infinity Stones at the end of Infinity War, and none of the starring characters in Avengers 4 (not even the soon-to-be-introduced Captain Marvel) are capable of traveling through time on their own. So they would need to find another powerful device or artifact that could take them through the time stream. Of course, the thing about time travel is just because you successfully alter something in the past doesn't mean the present will revert to being fully "normal" when you return. Regardless, time travel would certainly be cool to see in Avengers 4, but there may be a more plausible explanation for what's happening in these photos.
Before Captain America: Civil War main story kicked off, Tony Stark was shown demonstrating his Binary Augmented Retro-Framing, a.k.a. B.A.R.F., technology at MIT, which is able to project a user's memories as holograms. This technology didn't come into play again during Civil War, but when asked about if time travel will factor into Avengers 4, directors Joe and Anthony Russo hinted that these scenes set in the past are actually a result of the B.A.R.F. technology. This was further strengthened when other set photos showed an orange case with a "Binary Augmented Retro-Framing" label. It's also worth mentioning that at the same time that Crossbones was first rumored to return for Avengers 4, it was also mentioned that there will be a scene showing Tony studying The Battle of New York using B.A.R.F. with Harley Keener, the kid who helped him out in Iron Man 3. So assuming that B.A.R.F. is indeed in Avengers 4, that means that rather than going back in time, Tony and the surviving heroes are using B.A.R.F. to holographically display their memories of past events, presumably looking for clues on how they might be able to undo Thanos' actions.
There's one last thing worth bringing up regarding Avengers 4: it might be jumping forward many years. Back in April, it was reported that Emma Furhman had been hired to play Cassie Lang, Scott Lang's daughter. Cassie was played by Abby Ryder Fortson in the first two Ant-Man movies, and since Furhman is 16 while Fortson is 10, that suggests that we'll see Cassie as a teenager. Then last month, Gwyneth Paltrow seemingly let slip that in Avengers 4, Pepper Potts and Tony Stark are have a child together "this decade later," meaning that either Pepper was actually pregnant during Avengers: Infinity War or became so soon after. So rather than taking place only a couple months or even a few years after Infinity War, Avengers 4 could possibly reunite us with the surviving characters approximately 10 years later, living their lives on this new Earth where half the population was snuffed out of existence within seconds. This doesn't clarify whether time travel or B.A.R.F. is responsible for these scenes set in the past, but it does present an interesting predicament. If the MCU heroes are able to undo Thanos' genocide, that would also mean everything that had happened post-Infinity War being changed as well. So for Tony and Pepper, to bring all these people back to life, there's a strong chance they'd have to lose their kid. To borrow from Star Trek, it comes down to balancing the needs of the many versus the needs of the few or the one.