I have another idea. How about a 3D monopoly game?
Long time ago I was dreaming to make a game which starts when cavemen lived on Earth and You (player) have a couple of cavemens. They build houses, researches variuous things, upgrades everything. You own only 1 city. Your city is growing, and you continue playing until year 2000 (or similiar). You are involved into some wars (you must send some soldiers to the country's capital), etc, etc...
But thats imposible for me, so I made a small example of 3D monopoly game: