Well, with the in-game cars (a lot of late game potential seems to be wasted):
It's a very long article, so behold:
Kei Sports (difficulty: Low) - All 3 cars can be obtained as prize cars, obviously early-game.
- Honda Beat
- Daihatsu Copen
- Honda S660
Audi (difficulty: Low-Medium) - I will argue that a significant contribution of Audi in the history of the 'sports car' was to make them more practical and livable, hence the car choices.
- Audi TT 3.2 quattro
- Audi TTS
- Audi R8 4.2 FSI quattro
Volkswagen GTI (difficulty: Low)
- Volkswagen Polo GTI
- Volkswagen Golf GTI Mk7
- Volkswagen Golf GTI Mk1
Mazda Rotary (difficulty: Medium) - RX-7 FC would potentially appear as a prize car (waiting it to drop in the UCD just for this would be cruel), the others could be bought in the Brand Central. FD is pretty expensive though.
- Mazda RX-7 FC GT-X
- Mazda RX-7 FD Spirit R
- Mazda RX-8 Spirit R
Nissan Fairlady (difficulty: Medium) - Z432 should be a prize car, similar to the Mustang Boss 429 from the Mustang series, otherwise it becomes very difficult to complete. Also thought on whether the Z34 or Z33 would be better but decided on that the Z33 would better represent the history, even though the Z34 is more current.
- Nissan Fairlady Z432
- Nissan Fairlady 300ZX TT 2seater
- Nissan Fairlady Z33 (repeating car from Nissan Sports Cars)
Classic Muscle (difficulty: Low-Medium) - Unlocked after completion of 2 collection of Mustang vs Camaro. Menu Book 25 would potentially give the Challenger for free too! Otherwise the Challenger would be available as a Prize car somewhere else in America.
- Dodge Challenger R/T 1970
- Chevrolet Camaro Z28 1969
- Ford Mustang Mach 1 1971
Modern Muscle (difficulty: Low-Medium) - The GT350R is not repeated in the Ford Mustang collection.
- Ford Shelby GT350R
- Chevrolet Camaro ZL1 1LE
- Dodge Charger SRT Hellcat
Dodge (difficulty: Medium) - The pinnacle of "American Muscle" for me, even though the Viper isn't; that said, despite the supercar status, the Viper's brute-force approach, building the car around the engine, is uncommon for most European and Japanese sports cars.
- Dodge Challenger R/T 1970
- Dodge Charger SRT Hellcat
- Dodge Viper GTS 2013
Japanese 90s Legends (difficulty: High) - The difficulty of this obviously has to do with procuring the NSX, which is very expensive and pops up randomly as a used car. Other 2 cars are found in existing collections.
- Honda NSX Type R (1992MY only, not the 2002MY one from Brand Central)
- Nissan Skyline GT-R R33 (repeating car from GT-R collection)
- Toyota Supra RZ '97 (repeating car from Supra collection)
American Supercars (difficulty: Medium) - The C6 ZR1 is the only one not belonging to any other collection, but it can easily be obtained in the Brand Central.
- Dodge Viper GTS 2013
- Chevrolet Corvette C6 ZR1
- Ford GT 2006
Euro Sports (difficulty: Medium-High) - Two of them can be obtained for free if you obtain the Porsche 997 in the Pan-Am series. Huracan must be bought from the Brand Central.
- Ferrari 458
- Lamborghini Huracan LP610-4
- Porsche 997 GT3
Lamborghini (difficulty: High) - There are so many awesome Lambos in this game, but I will say that the Countach, the Huracan, and the Aventador SV would be the most significant ones that are featured in the game.
- Lamborghini Countach LP400
- Lamborghini Huracan LP610-4
- Lamborghini Aventador SV750-4
Hypercars (difficulty: Extreme) - Requires invitations for all 3 cars hence the difficulty rating.
- Bugatti Veyron 16.4
- Pagani Huayra
- LaFerrari
Track Hypercars (difficulty: Extreme)
- McLaren P1 GTR
- Pagani Zonda R
- Aston Martin Vulcan
Le Mans Champions (difficulty: High-Extreme) - Perhaps easier than Hypercars due to all 3 cars being acquired through the Brand Central
- Peugeot 908 HDI FAP (2009 winner)
- Audi R18 TDI (2011 winner)
- Toyota TS050 (2018~2020 winner)
Next-Level Racing (difficulty: High-Extreme) - Others might have different opinions on how VGTs should not compete in Group 1 or what types of VGT were the most outlandish, but I would say those 3 would best describe the word "Next-Level Racing"
- McLaren Ultimate Vision GT Gr. 1
- Mazda LM55 Vision GT Gr. 1
- SRT Tomahawk Vision GT Gr. 1
Limit Break (difficulty: High-Extreme) - Featuring 3 of the fastest cars in this game
- SRT Tomahawk X Vision GT
- Gran Turismo X2019 Competition
- Chaparral 2X Vision GT
Group C Legends (difficulty: Extreme) - Sorry Mercedes fans, that the C9 '89 couldn't make it in, but 787B and XJR9 were more significant competitors
- Mazda 787B (1991 winner)
- Jaguar XJR-9 (1988 winner)
- Porsche 962 (multiple crown winner)
Gran Turismo Champions (difficulty: Medium-High) - represents the Manufacturers' series champions of GT Sport
- Subaru WRX STI Gr. 3 (2020 winner)
- Toyota GR Supra Racing Concept Gr. 3 (2019, 2021 winner)
- Lexus RC F GT3 Gr. 3 (2018 winner)
1950s Racing Legends (difficulty: Extreme) - All of them are Legends
- Mercedes-Benz 300SL CN.194
- Jaguar D-Type
- Aston Martin DB3S CN.1
1960s Racing Legends (difficulty: Extreme) - Quite possibly one of the hardest achievements in this game if implemented.
- Ford Mk. IV - the Mk. I "road car" won't count.
- Ferrari 330 P4
- Porsche 917K
Two more collections, if we could add 1 car:
1950s Legends (difficulty: Extreme)
- Mercedes-Benz 300SL
- Porsche 356 Speedster (Coupe not eligible)
- Corvette C1 '59 (this one is new, significant in Corvette history... Sure we have the Stingray Racer '59, but normal C1 looks better. A similar model was present in GT6 as the Art Morrison Corvette'60)
Lightweights (difficulty: Medium)
- KTM X-Bow R
- Radical SR3
- Lotus 3-Eleven (If Lotus was here we would've got this)
And I honestly forgot all about S2000, that'd replace Barchetta then.
