This isn't really a car, more of a feature but it's related to cars. I would like all racing car duplicates to be just added to one car. (When buying a race car, instead of there being 4 duplicates of the same car, there would be one car and you could select from the 4 liveries.)
As for cars, I'd love to see more Le Mans GT1/2/3/C cars, as well as more modern LMPs. But more importantly Group C cars, god I love them. (Some notables being Nissan P35, Mazda 767, Lancia LC2 and so on.)
I'd also like to see more Ford V8 Race cars and Aussie Race and Sports cars. I wouldn't mind a Daytona Prototype either.
Tracks? Nardo Ring, SSR11, and the SSRs in GT5 as well. Also that one street track in GT4. I can't remember the name but it was featured in a Special Event and you won the CIEN for it.