[SimHacker]

Well, Spore is a multi layer game, but it's "Massively single player", and it didn't manage to couple the layers as well as was originally hoped.

One problem is that the different layers want to run at different time scales, so you have to decouple the flow of time of each layer, and somehow reconcile things when you move up and down between them. And of course a multi player game can't have the same clock for everybody, so players will get out of synch with each other. Spore avoids that problem by not synchronizing player time, just sharing user created content asynchronously.

[pessimizer]

Wow, time would be a big problem when designing something like that. You'd either have to decouple or market the different games to different types of players - for example a slow 20 minute a day empire-building game layered on top of an addictive grinding World of Warcrack type thing. Something to think about.

Thanks for the slicecity link.