Welcome,
Well this is actually pretty old vision of ours too since when we got together. It's still just a vision though - army of devils is in the implementation details.
But our idea is slightly different - instead of making multiple clients we would make a plugin architecture where specialized simulators could be written by other parties, handling physics and systems simulation. It should then be possible to use a specific simulator plugin with an appropriate vehicle or device, while for other players the machine behaves like a black-box controlled by its internal forces propagated over the network so that everybody doesn't have to have all the plugins but the behavior is in sync.
And this way there could be just one client that handles the world&object rendering, as this will be the common part anyway.
It would be the real Earth, but I'd like to have different "realms" that contain theme-specific settings and assets. I don't think fantasy or sci-fi setting should mix with the real one, and there even could be multiple real ones from different ages. Or different games could use their own specific realm initially derived from a basic one but changing in the course of the game.
