There are a few cars I'd like to see in the game.
Most Interesting:
2005 Chevy 
C6 Corvette
1991 Chevy 
S10 Syclone
2006 Dodge 
Magnum Police edition
2004 Dodge 
SRT-10
2004 Dodge 
SRT-4 Neon Extreme Lightweight
~2004 Mazda 
RX-8 turbo concept.  Very cool concept from Mazda Australia.
Silly, but interesting:
2004 Rolls Royce 
Phantom (6.75 litre V12)
2005 Bently 
Arnage (6.5L V8)
And of course the various GM 
concept cars from days gone by.
Historical cars that are interesting:
MGB
Triumph 
TR6
1966 Lamborghini Miura fastest car in 1966, 352hp V12, 170mph
1987 Ferrari F40
1992 Yamaha 
OX99-11 I remember reading about this and the McLaren F1 at the same time as they were both rather similar.
Super cars:
2004 Bugatti 
Veyron 1000hp sounds fun.
2004 Koenigsegg 
CCR Possibly the fastest production car in the world.
1997 McLaren 
F1  Getting old, but can't be beat.
1996 Vector 
M12 (or WX3 or W8) Vector deserves to be in there!
2002 Mosler 
MT900 Perhaps not the highest top speed in the world, but likely the quickest around a track!
2004 prototype Chrysler 
ME Four-Twelve This is going to be one quick car!
200x Ferrari Enzo, etc
Lamborghini Diablo GTR, etc
I know the reasons for there are licensing issues to contend with for at least three of those brands, but some of those cars could make it into the game.  I'd rather see some of those different vehicles than 15 very similar Subaru WRX STi cars.  It's a great car, but I'd rather see some others in the game before getting 14 additional variations on it. 
