Another game I tried to play with solo was Godsend. I really liked the idea of using Gods and Avatars to create a world; a campaign before a campaign, if you will. Unfortunately, it was not something that I could run easily with my group. So I tried solo playing it for a while; I made two gods and two avatars, and would flip between making decisions about the world and making decisions about the characters whenever I picked up the game. I still have the folded-up map that I scribbled over and changed every time the avatars got into trouble.
I ended up running out of steam on this one, but I think it would be an interesting game to return to.