CAUTION THIS FAN THEORY MAY SPOIL SOME POINTS IN INFINITY WAR.
There have been several different theories about the next Avengers movie since the cliffhanger at the end of Avengers: Infinity War.
One of the theories is the time travel theory.
Many believe that Scott Lang AKA Ant-Man will find that his daughter has turned to ash, possibly during a post-credit scene in Ant-Man and the Wasp.
During the Ant-Man and the Wasp movie, they are supposed to be utilizing the quantum realm to find Janet (the original Wasp). In the quantum realm time and space does not exist thus allowing the users to time travel (in theory) to a different time.
Some Avengers 4 set photos have emerged showing a present-day Tony Stark and Ant-Man with a Captin America in his Avengers (2012) uniform.
It seems as though Tony and Scott at some point meet up and travel back in time to stop Loki before his assault on New York. In one of the photos, you can see Loki looks like he was captured. They may apprehend him before he gets to summon the Chitauri army.
In Avengers Infinity War, Tony never reunites with Steve during the battle to stop Thanos. Tony gets abandoned on Titan with Nebula after the “finger snap” most of the cast disintegrates to nonexistence. He probably can’t use his suit to travel the long distance back to earth however the Guardians ship should be still on Titian and he and Nebula could use it to get back to Earth.
Another theory is that Avengers 4 takes place 10 years after the events of Infinity War, and its said that the team unable to locate Thanos decided to go on with their lives after half the universe is wiped out. This also comes from Gweneth Paltrow her self in an interview where she said that Pepper and Tony have a kid in Avengers 4. This explains the time jumping. Unless Cap goes back to his old uniform, and Thor grows his hair back. That still doesn’t explain much about why they would do that. What about the Sokovia Accords? Cap would still be a fugitive, they only slightly reminded us of that problem during a moment in Infinity War when Cap, Widow, Falcon, Vision, and Wanda arrive back at Avengers HQ and Rody was talking to the court marshall.
It could be that they travel back to 2012 for the attack on New York to stop Loki.
Or they could start Avengers 4 ten years later with Ant-Man and the whole quantum realm time travel issue, but I cant speculate on that any more until after I see Ant-Man and the Wasp.
They have to put the universe back to how it was because both Black Panther and Spider-Man are getting sequels, and they have already begun adding new heroes with Captain Marvel joining the team in March.
Currently, Captain Marvel is the only newest team member joining the MCU at this time, and Marvel Studios are already in talks of adding more movies that will span out until 2025 according to Kevin Feige.
Either way, only time will tell exactly how Avengers 4 will unfold, but it’s supposed to be the movie that changes the MCU as we know it now.