[BRAINSTORM] Online and social of cooperative multiplayer


I have kids at home who are very engaged with roblox (www.roblox.com) and they can play everyday for hours without getting bored.
Therefore, I have been thinking of setting up something similar but maybe for grown up (teenagers etc) players.

First idea that I can think of is setting up real cities buildings and then invite people all over to download games binaries and participate (it can be a fun stuff eg treasure hunts and the victor will get virtual money etc).

So at this point, idea is very sketchy and raw but I think this is very interesting project to pursue. I am not looking for technical inputs (like the backend, server shards etc - that will come later). But I am looking for

  1. similar social 3d games (or 3d if there is any)
  2. other ideas - I have only one idea so far…

Thanks for all feedbacks.