Well, rather than writing down every car I dream of, I'll stick with the Manufacturers we already have, so Ubi would be able to obtain licenses for those vehicles.

Abarth:

124 Spider (the new one based on the MX-5)

Alfa Romeo:

33 Stradale
155 Q4 (4WD) also the DTM Racecar as circuit spec.
Brera
GT
Giulia
Giulietta
Stelvio

Aston Martin:

Vulcan
V12 Vantage
Rapide
Bulldog
One-77
Aston Martin DBR1-2*
AMR-1*

Bentley:
Bentley Turbo RT Mulliner
2003 Speed 8*

BMW:
M1 (M1 Procar as Circuit spec)
Z8
E46 M3 GTR Street Version (ALMS M3 GTR as Circuit Spec)
E31 8-Series (Wagenstetter Motorsport used one in the VLN Nordschleife racing series, so a circuit or perf spec could be based on it)
E30 M3 Sport Evolution
E92 M3 (also the GT2 Version which raced in the ALMS)
BMW Nazca C2 (not C2 spider).. some people might remember it from old NFS Titles

Cadillac:
Cien Concept '02(don't we all remember the the Easymode for Gran Turismo 4?)
CTS-V
Sixteen

Chevrolet:
'77 Aerovette Concept
'89 Camaro IROC-Z28

Chrysler:
Conquest TSI (cheesy way to get a Mitsubishi Starion)
Prowler (also a Car some of us might remember from old Games)
Turbine Car (Concept)
ME-Four-Twelve (Concept)

Dodge:
Viper SR I GTS-R. Trivia: In 1999 following new regulations for the Series around the 24h Nürburgring, the Zakspeed Team entered a Viper build to GT2 Spec by Oreca, which was completely dominating the Race. The car had a huge impact on the Series and attracted a lot of Fans. Here are some pictures of the original design: http://www.autoartmodels.de/de/dodge...-gts-r-99.html

Ferrari:
F50
Testarossa
Mythos (Concept)
308 GTS Quattrovole

Ford:
GT90 (Concept)
Sierra RS Cosworth
GT40
RS200

Hummer:
HX (Concept)

Jeep:
Grand Cherokee Trackhawk

Koenigsegg:
CCX and the CCGT as Circuit spec (GT1 Racecar that never raced because GT1 regulations were changed just as it was finished, Königsegg would have to have sold 350 CCX to be permitted to start it).

KTM:
X-Bow RR

Lamborghini:
Countach LP500 Turbo S
Diablo GTR (also the Circuit Version from the factory racing series... one also raced in the Bathurst 24h in 2003).
Reventon
Sesto Elemento
Egoista (Concept)
Veneno
Centario

Lotus:
Esprit (and GT1 version for Circuit spec)
Gen 1 Lotus Elise (and Elise GT1 for circuit spec)
3-Eleven

Maserati:
Birdcage 75th (concept)

Mazda:
MX-5/Miata NA
787b*
RT24-P*

McLaren:
M6GT
P1

Mercedes-Benz:
190 Evo II
300SL
CLK GTR

Mini:
one of the "original" Minis

Nissan:
GT-R Nismo N-Attack
350z
R390 GT1 Road Car (Race Version in Circuit spec)
R32 GTR
300ZX

Pagani:
Zonda Revolución

RUF:
Dakara (Cheyenne)
BTR
CTR "Yellow Bird"
CTR2
RGT-8

Saleen:
GTX (Tesla Model S)

Shelby:
Super Snake 2017 Anniversary edition

Spyker:
C12

TVR:
Cerbera Speed 12
Tamora
Griffith
Chimaera

Volkswagen:
W12 Coupe aka W12 Nardo
Polo (iirc they currently dominate the WRC)
Golf VI (They build a Golf24 for the Nürburgring 24h race, which would make a decent perf or circuit car)