Perhaps not a full MMORPG, but you could do something like an asymmetric VR game (one player has a headset and the others watch and guide them via a regular computer screen). Something like Keep Talking and Nobody Explodes, Acron Attack Of The Squirrels, Davigo, Panoptic, etc. You could have a lobby system (the multiplayer part) where new players find people to create a team and maybe other players can choose to watch games while they aren't active. They compete as individuals and/or teams, but while in a game they are alone in the dungeons.