Super cool site. I just imported all my data for my Honda Fit. I'd like to suggest tracking total cost of ownership data. This would include purchase price, registration, insurance, maintenance, gas, repairs etc..
The number can be displayed in Total Cost of Ownership as well as Cost of Ownership Per Mile. Maybe even have contests for lowest cost of ownership car.
Right now we're staying focused on fuel economy, but we get frequent requests to expand our Notes feature into a full-blown maintenance helper with reminders and suggested maintenance plans. Adding features to track every aspect of owning a vehicle is a bit out of our scope right now, but I do think there would be interest in it.