Adrenaline Vault has an article on the sociological aspects of driving games. It's pretty interesting.

Driving games have always been about the fulfillment of socially sanctioned male fantasies.

I would expand that to: games are about the fulfillment of fantasies. Forget "socially sanctioned": transgression is a powerful fantasy, and therefore a major cause of controversy. GTA3 is an obvious example, but think of Tony Hawk: you ride around places you're not supposed to (high schools, airports), break stuff, and freak out the squares. The game I worked on most recently fulfills a common fantasy of taking revenge on an annoying neighbor.

Identify people's dreams, illicit or not, and you can create a compelling interactive experience for them. (It's also a good way of thinking about game development: a dream is a high-level concept that you can use to decide what to do and what not.)

Go to a good news agent and look at the magazines, especially those aimed at hobbyists. Identify the expensive hobbies (preferrably involving a little technology so that there is no fear of PCs or consoles): there's your market. Railroad Tycoon appeals to people who like model railroads, Soldier of Fortune appeals to gun nuts. Perhaps there are some untapped market segments right around the corner.