So like what they did with GTA5, but set in the Marvel Universe?
Yeah, that would be amazing.
Yeah, and then there's side missions that are optional to do.
My roster would be:
Iron Man (based on Stark Building)
Captain America (based on hellicarrier)
Thor (based on Asgard)
Hulk (based on an university)
Ant-Man (Hank Pym, based on quantum realm)
Wasp (based on quantum realm)
Black Widow (based on Shield's land HQ)
Hawkeye (based on Avengers mansion)
Doctor Strange (based on Sanctum Sontorum)
Spider-Man (based on Queens)
Black Panther (based on Wakanda)
Captain Marvel (based on Shield's space hq)
Shang Chi (based on a dojo)
So that's thirteen. I added Wasp so theres one more female playable character. There won't be a playable character that is absent in the cutscenes. Each have their own role in the main story, everyone will have their own mission. Nick Fury, Falcon, Agent 13, Vision, Heimdall, Sif, Warriors Three as npcs, some would assist during a mission. Winter Soldier, Green Goblin, the Leader, Baron Mordo, Loki, Yon-Rogg as villains, and a lot of the villains would be under Masters of Evil. I would save Quicksilver and Scarlet Witch for the X-Men. Easter eggs for the other superhero teams would be scattered throughout the game. Certain characters (those who can't fly or swing) have a vehicle that they can use especially for free roaming.
Quantum realm, Wakanda, Asgard won't be that massive, since the game I imagine would be set mostly in New York. But the hideout in those locations, would be as big as Ezio's hideout in Ac2. Then of course there would be story missions in those locations as well, so as side missions. Also lots of collectibles scattered throughout the entire map.
There's really a lot they can do in an Avengers open world game.