I think there should be a horse mode.
Before you press the "back" button, hear me out.
Like cars, horses are a popular mode of transport.
However, I don't think there is a game in the market for customised horse racing. I think there should be a mode in GT5 where you can race horses around the circuits.
PD could use this to appeal to me and the millions of horse lovers around the world. This could even create a whole new market of gamers, who have bought a console solely for the horses.
I absolutely love horses, and this would add a lot of variety to the game.
Edit: Forgot to specify that it would be a bad idea to race cars and in the same race, so I think horse mode should be wholly separate from the car bit.
Instead of changing oil you could buy more hay.
Edit: And you could race horses around the circuits that the cars race on.
I think it would be good if you had to unlock this mode too.