The good news is I came up with an idea that will work a whole lot better. Think MMO GTA 1 without people, or possibly Smash Bandits Racing - but really ugly - where you instead can build your own house and vehicle. Your lawn can house a cemetery or a skyscraper. Your vehicle could be a bus or a space ship. Mount canons. Voilà!

But exactly how difficult is this beast going to be to develop? Hm. I need two really good in-game editors. A pre-generated, enormous city. The MMO servers would have to be offloaded to the users, because if the game becomes really huge I couldn't afford the server cost. Everything would have to synchronize all the time - including all estates and vehicles. The estates and vehicles would have to be saved on some central database, with user accounts connected. Local caching to reduce network traffic and lag.
Last but not least the gameplay itself must rock insanely.
Something tells me I have some work ahead of me, but I'm a lot more confident about this idea than I've been about any gaming idea lately. Better start working on it before I find a flaw!