RPGs are always a good choice, as they keep players addicted. What about a "survive in an island" type of game? Or a city roleplay, or anything. Your imagination is the limit, don't forget that.
Do a loop the loop.
SMMORPG defence against dinosaurs with jetpacks riding laser-powered sharks which eat water, drink uranium and breathe radiation strapped to stars with rocket engines which shoot rainbows.
Now go make it.