The Russo Brothers have done a fantastic job in the MCU thus far, with great movies like Captain America: Winter Soldier, Captain America: Civil War, and Avengers: Infinity War under their belt. Avengers 4 is set to be their swansong for now, not just from Marvel but superhero movies in general, at least until one specific Marvel story is ever considered for adaptation.
During a Q and A hosted by Collider that took place after a special screening of Infinity War, The Russos claimed that they would be done with superhero movies until Secret Wars. No, not the epic 2015 event from Jonathan Hickman and Esad Ribic but the original 1985 crossover that was the basis for all future Marvel comic book events.
Secret Wars has a pretty basic premise; a cosmic entity known as The Beyonder gets some of the Earth's most powerful superpowered beings and forces them to take each other on. They are placed into two groups based on their ideals so there was a team for heroes and villains. It did have a few neat twists though, like adding Magneto to the group of heroes since his motivations aren't that black and white.
Considering how influential and fun the event was back then, it's easy to see why they would want to direct a live-action adaptation of this movie. If the Fox deal goes through, we could finally get the X-Men, Fantastic Four, and Avengers in one cinematic universe and The Russos want dibs when that happens.
Avengers 4 comes out on May 3, 2019.