Avengers 5 is probably, at the very least, a few years away. With Phase 4 of the Marvel Cinematic Universe not even set to kick off until November with the release of Black Widow, it is tough to speculate as to what might happen in the eventual sequel. That said, a new fan theory has emerged that suggests Hulk will become the bad guy, essentially turning into a World War Hulk adaptation.
The theory was shared on r/FanTheories Reddit page by user u/smileimhigh. The post posits that Avengers 5 will be a loose adaptation of the World War Hulk storyline. It boils down to Mark Ruffalo's Bruce Banner harboring some issues related to the events of Avengers: Endgame. Those ill-feelings will, per this theory, cause Hulk to take the driver's seat once again, resulting in the Avengers needing to take down Hulk. The theory reads as follows.
"I don't have much proof so this isn't the strongest theory but.... Banner destroyed his arm and Natasha died, then Tony gets all the credit for the victory when it was Hulk who snapped and brought everyone back. Banner has anger issues, despite Ruffalo's version being chill, it's still in his history. I see it like this: Banner gets depressed, starts thinking if only I was as strong as I was on Sakaar I could have snapped and brought Natasha back and/or defeated Thanos and save her from having to sacrifice herself. This leads to the 'Devil' Hulk coming out which in the MCU will basically be a vengeful violent version of the super strong World Breaker Hulk brought on by Banner's guilt and anger."
For those unfamiliar, World War Hulk was published by Marvel Comics in 2007 and picks up in the aftermath of Planet Hulk. This storyline was loosely adapted in Thor: Ragnarok. The follow-up sees Hulk returning to Earth to wage war on his fellow heroes as they banished him to space in the first place. It turns into an all-out, destructive war as Iron Man, Reed Richards, Doctor Strange and Black Bolt are on the wrong end of Hulk's rage.
In the MCU, Bruce Banner returned to Earth with no ill-will towards the Avengers following his stay on Sakaar. Banner warned them about the impending arrival of Thanos and ultimately reconciled the two sides of his personality, as we saw in Endgame. This theory suggests that the delicate balance is very much capable of being interrupted.
For now, all that has been said officially regarding Avengers 5 is that it will happen and that it will feature a very different team, per Marvel Studios head Kevin Feige. As for when we could see it? Disney has its Marvel slate booked up through 2022. At the very earliest, it would be sometime in 2023. So we've got some time to see if this theory has any truth to it. This comes to us via Reddit.