I know it's utterly unworkable. But I want it anyway.
Imagine an RTS game that ... just keeps going. Both in time and in play area. Something like the Minecraft map in scale.
You play a few hours online with other people, log off, come back, and then you're still playing on the same map with the same resources, buildings, and units. Other people may have advanced and tech'd up, and now you can too.
I have no idea how to handle combat when a player is sleeping or making dinner. Or any other real conflicts. Maybe a timer of some sort? Maybe catching them sleeping is part of the fun?