I would really like to see an all-in-one track that would take care of a lot of these requests. That way we don't end up with a dozen test tracks, and Eric would have to do less work. Here's what I'd like to see:
A very large oval similar to the one in Gran Turismo. This would allow for top speed testing and using the straights would be good for 0-60, 0-100, 0-100-0, half-mile, etc. Inside the big oval would be a smaller, half mile dirt oval (hopefully we'd eventually see a stock car or sprint car to go with it). Inside the dirt oval would be an off-road "stadium racing" style track that's all dirt. This would be great fun in the current RallyX car setups, but would be even better with an off-road truck.
This proposal shouldn't really need any new textures, and the modelling should be quite easy compared to any of the other tracks. Most of the textures and models could probably be imported from other tracks (like Kyoto) with only slight changes.