I'm a Porsche-file, so anything Porsche would be great if Polyphony could score the licenses for GT4. Of course I don't want to loose the RUFs in the process. But, as others have mentioned:
959
911 Turbo S (I prefer the 993 type but some of the older 965 and newer 996 types would be cool too.)
968 Turbo S/
928
Bring back the BTR, CTR, Turbo R, RGT, CTR, CTR 2, CTR 2 Sport
Maybe some Strosek and TechArt 911s
DP 962
961
GT1, GT1R, GT2
I'd also like to see:
Volkswagen
W12 Concept
Jetta (1.8T Oettinger)
Passat (1.8 T Oettinger, W8)
GTI
Golf (1.8T Oettinger)
Mercedes
SLK 32 AMG
CLK 55 AMG
CLK GTR
Volvo
S80
Lamborghini
Murcielago
Diablo
Countach
Ferrari
F-40
F-50
575 M Maranello
Enzo
F355 Spider