Avengers 5, for now, is just an idea among fans. There is no word on when Marvel's heroes will unite for a massive crossover event and giving the idea some time to rest might be a good idea. After all, going bigger than Avengers: Endgame could result in a massive mess and the scale of the film calls for a bit of time to breathe. However, when a fifth Avengers movie does inevitably come to pass, there is a new theory which combines the events of Avengers: Endgame and a popular Marvel Comics story and features the Hulk as the main villain.
The theory surfaced on Reddit, the home of endlessly entertaining speculation , theories, fan art, and other Marvel content. Reddit user smileimhigh (first of all, what a name, right?) posed the idea of Hulk growing resentful towards Tony Stark after his sacrifice to defeat Thanos and save the world was famed and remembered. Meanwhile, it was Hulk who snapped his fingers and endured the effects of the Infinity Stones to bring everyone back to life. Pair that anger or jealousy in being overshadowed by Stark with the death of his once love interest Natasha Romanoff and the Hulk might not be too happy with his pals. They didn't even give her a funeral.
This is where the World War Hulk story comes into play.
In the Marvel Comics story, the Hulk is banished from Earth. The Illuminati, a group which included Doctor Strange and Iron Man and the time, are among those who cast Hulk out. The big green monster loses his wife while jetting off of the planet, prompting a fiery rage directed at those who sent him away. After spending time on Sakaar, Hulk has become stronger and heads back to Earth to exact revenge.
While there is no Hulk movie in development, Mark Ruffalo has several times over teased involvement in the upcoming She-Hulk series on Disney+. Perhaps as Jennifer Walters develops her She-Hulk powers, Ruffalo's Hulk will become unhinged in the aftermath of Avengers: Endgame and ultimately lead to a war on his home planet against his former friends?
Some of the seeds for these story threads to grow are in place but it can definitely use a bit of extra work in its lead up to make sense and feel fitting but the idea of a World War Hulk story being the reason for an Avengers 5 or, at least, a subplot of the film is an exciting notion for Marvel Comics fans.