I know Kaz has a very strong sense of national pride (or rather Nissan pride) but I really wish he would try to incorporate parts of other country's car cultures. The SEMA and Pebble Beach cars are some of the most interesting in GT5. As much as I like SGT cars, I would rather he add cars that are unique and we have rarely see.
Like:
Classic Racer Pack
Ferrari 32P
Lamborghini Countach Race Car
Ford Mustang Boss 302 Parnelli Jones Car
Beetle Rally Car
Toyota 2000GT Race Car (driven b Carrol Shelby no less)
Max Tune Pack (not exactly tuner cars but cars that are tuned to their max performance)
RUF GT9R
Mercedes GLK Hill Climb Car
Alpha 9 GTR
Hennessey Venom GT
Concept Pack
Ferrari P4/5
NSX before the current redisgn (how cool would it be to drive a car that WILL NEVER be made!)
SAAB AeroX
Renault Espace F1 Concept Van
Not that I have anything against what we have gotten so far... but I find what makes GT different is driving cars you have never heard of before and realizing how amazing they are. Before the GTR, Evo, etc became main stays in the US, people discovered them through Gran Turismo.