Basically I would love a game where players HAVE TO cooperate. The cooperation happens by having players playing different gameplays.
For instance, but, keep in mind that this is just ONE example: there's a war to fight, some players are drivers, some are fighters, some are medics...But also some players are in offices doing strategy stuff or logistics stuff, etc.
Of course, one could say: "you can have that in Arma 3 or Foxhole" and that's kinda true...but I don't know, there's something missing: A lore, a story, a universe to feel part of...
Plus, the game I am imagining, does NOT have to be a simulation or "complex gaming" at all! It could be kinda-casual without being too simple either: like For The King.
So in my ideal version of the game I am thinking about players play actually VERY DIFFERENT gameplays but cooperate in order to achieve common goals. Some of the players have to interact, some of the players don't. But in any case, everyone's actions will impact everybody else.
To sum it up: I would love a game with: Persistent universe + very asymmetric gameplay + cooperation as a key factor of success
But the game would be without: Grinding, farming, loot boxes, meaningless quests, basically a game where you don't have to redo the same things on and on to grind 128points of experience or reputation or obtain one piece of loot that does not fit your equipment set.
If you think this game already exists: do not hesitate to tell me :)