Instead of just giving a list, I'll actually justify why those cars should be in the game. More than anything I think GT5 needs three companies whose names start with F, P, and L, but what can you do? If EA can manage to tie up entire professional sports leagues for years I'm sure they can tie up the coolest cars too.
Most Interesting:
2005 Chevy
C6 Corvette. OK, this car is obvious.
1991 Chevy
S10 Syclone. First of the fast pick ups I can think of. AWD to beat super cars off the line.
2006 Dodge
Magnum Police edition. I'd like to see some police cars in the game and I'd like to see some Hemis...
2004 Dodge
SRT-10. Big engine, big vehicle. Kind of interesting. At least it could demonstrate that trucks aren't the new muscle cars. Even though they've got big power they aren't that fast.
2004 Dodge
SRT-4 Neon Extreme Lightweight. Nice little car, very fast too. A nice alternative to having a dozen variations on a Civic. I doubt you'll find a faster FWD, American or otherwise.
~2004 Mazda
RX-8 turbo concept. Very cool concept from Mazda Australia. The RX8 is a great car in real life for practical reasons with four doors and on the track it is great too I can't think of a better balanced car out there at any price range. However it only makes ~240 hp which just doesn't cut it against the higher power cars. So this concept gives it some real power!
Silly, but interesting:
2004 Rolls Royce
Phantom (6.75 litre V12)
2005 Bently
Arnage (6.5L V8)
Big engines are fun.
People are always concerned about horsepower rather than power to weight ratios so give them these two boats and see how they do.
If we want to get even more silly there are of course the various GM
concept cars from days gone by. Can't say I really want to drive one, but I'd rather be passing one of those than another generic looking car.
Historical cars that are interesting:
MGB. It's a classic!
Triumph
TR6. Classic.
1967 Lamborghini
Miura fastest car in 1966, 352hp V12, 170mph
1987 Ferrari F40. Such a nice car. Hard to believe how old it is.
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 would've been one quick car! It sounds like it was ALMOST going to be produced too!
Of course:
200x Ferrari Enzo, etc
Lamborghini Diablo GTR, etc
Those are just obvious though...
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.
I'd also like to be able to race against both a friend and some computer cars.