I'd have a couple of ideas. One is Ultimate Nascar, and it's complicated, but I'll try to explain it. Imagine a big racing oval, except on the long parts of the oval (on both sides) there are football fields. People play football on these, except they're armed and allowed to kill whoever they want in the process of scoring a touchdown. All the while, racing vehicles with heavy guns speed around the track, killing each other as well as the people on the football field for points. And, just for fun, there's other random weapons strewn about the massive colisseum in which the whole event occurs, as well as sniping spots, and hidden secrets, etc... Add to this really fast respawn times, random events (alien invasions, zombie infestations, etc), and the fact that it also features an Oblivion style leveling system, so that you get experience for doing stuff in the colisseum. And, to top it off, it's an MMO. It will be chaotic, you'll have to find the secrets to uncover the plot, but if it wouldn't be fun I don't know what would.
Also, a top down RTS (think something like Age of Mythology in mechanic), I don't know if I'd want it to be a midieval theme, a futuristic theme, or maybe even a modern theme. Point is, you'd have the time to build up as much as you want. That's what always annoyed me about Age of Mythology, I was having fun sitting around building up my fortress and raising an army, but that silly enemy AI wouldn't leave me alone. Then after you've built up as much as you want you can go annhialate your enemy. It probably won't be too challenging, more like Sim City except you get to go kill people when you feel like you've built up enough.