An MMOFPS/RTS. Think Planetside, but then add in RTS elements, like the ability to command vehicles and construct buildings from above.
It's already been done on a small scale if you look at a game like Natural Selection. NS combined RTS and FPS perfectly. Another really interesting example is Dust 514, which combines the persistent environment of Eve Online with a brand new FPS game.
Oh, and awesome crafting abilities. Give us the ability to design all manner of devices, and even vehicles.
I wonder what Blizzard is up to with Project Titan. Could it possibly be what I just described? I only hope so.