Wow, lots of fancy race cars in this thread.
I prefer the road cars, and some more historic representation is needed from somewhere besides Japan in GT5. I am sure some of the more modern cars are in the works already, like the Cadillac and the new 2011 mustang.
I would add
Bricklin SV-1
I would also like to see more examples of Mustangs. A Fox body hatch to compete with some of the many 80's Japanese cars. Maybe a mid 90's Cobra. And especially the 580HP Super Stallion from the late 90's.
Some older "Nascar" cars also. I hate Nascar, personally, but in the late 60's and 70's it was a totally different race. Some of the old Mercury Cyclones to race with the Charger daytonas and superbirds on a nostalgic daytona track would be a lot of fun.
AMC
AMX, Pacer, Javelin. Something from the States that isn't big 3. The Matador they produced was competitive racing in Stock car races.