I always had an idea in my head for an MMO type game where people pilot a huge spaceship. Different people would play different roles:
Certain players control the cannons.
Other players control navigation, ship upgrades, and medical.
Still, other players would leave the ship and explore planets below.
I had dozens of other roles in mind. I even thought about massive "Star Destroyer vs. Star Destroyer" style battles where everyone involved was a real person. It's an impossible game to design, but it would be cool if someone could pull it off.