I’ll join the DLC wishlist/idea train as it looks rather fun actually. If I had to make some DLC packs, they would contain 6 to 12 cars (might be overkill) under a specific theme (as others have shown) which contain new and/or returning cars - here’s 10 car pack ideas:
Japanese Heroes (12 - Returns):
- Honda Integra Type-R (DC5) ‘04
- Honda S2000 ‘06
- Mazda Autozam AZ-1 ‘92
- Mazda RX-7 GT-X (FC) ‘90
- Mitsubishi GTO Twin Turbo ‘98
- Mitsubishi Lancer Evolution VIII MR GSR ‘04
- Nissan Skyline 2000GT-R (KPGC110) ‘73
- Nissan Silvia K’s AERO (S14) ‘93
- Subaru Impreza WRX STi (GD Type-II) ‘02
- Subaru Legacy B4 Blitzen ‘00
- Toyota Celica GT-FOUR (ST205) ‘98
- Toyota MR-2 Spyder ‘99
American Classic Muscle (8 - New/Returns):
- AMC Javelin AMX ‘72
- Buick Special ‘62
- Chevrolet El Camino SS 396 ‘67
- Dodge Charger 440 R/T ‘70
- Ford Torino Cobra ‘70
- Mercury Cougar XR-7 ’67
- Pontiac GTO “The Judge” ‘69
- Plymouth Cuda 440 Six Pack ‘71
American Modern Muscle (8 - New/Returns):
- Chevrolet Camaro SS ‘00
- Chevrolet Corvette Z06 (C5) ‘04
- Chevrolet Corvette Z06 (C6) ‘05
- Dodge Challenger SRT Hellcat ‘18
- Dodge Viper SRT10 ACR ‘08
- Ford Mustang Shelby GT500 ‘13
- Ford Mustang SVT Cobra R ‘00
- Shelby Series One Supercharged ‘03
European Hot Hatch Collection (10 - New/Returns):
- Abarth 695 Competizione ’16
- Alfa Romeo 147 GTA ‘02
- Mini Cooper S ‘11
- Mini Cooper S JCW GP ‘16
- Peugeot 207 GTI ‘07
- Renault Mégane R.S ‘08
- Vauxhall Corsa VXR ‘15
- Vauxhall Astra VXR ‘13
- Volkswagen Golf V GTI ‘05
- Volkswagen Golf VII R ‘18
JGTC Legends (6 - Returns):
- Honda Castrol Mugen NSX (JGTC) ‘00
- Honda weider HSV-010 (JGTC) ‘11
- Nissan Xanavi Nismo Z (JGTC) ‘06
- Nissan C-West Razo Silvia (JGTC) ‘01
- RE AMEMIYA AMEMIYA Asparadrink RX-7 (JGTC) ‘06
- Toyota Wedssport Celica (JGTC) ‘03
Festival of Ferrari (8 - New):
- Ferrari 159 S ‘47
- Ferrari 348 GTB ‘94
- Ferrari 360 Modena ‘99
- Ferrari 488 GTB ‘16
- Ferrari 599XX ‘12
- Ferrari 612 Scaglietti ‘05
- Ferrari 812 Superfast ‘19
- Ferrari F355 Berlinetta ‘98
Rally Legends (12 - New/Returns):
- Audi Sport Quattro S1 Rally Car ‘86
- Ford RS200 ‘84
- Ford RS200 Rally Car ‘85
- Lancia Delta S4 Rally Car ‘85
- Lancia Rally 037 Rally Car ‘83
- Lancia Stratos Rally Car ‘77
- Mini Cooper 1275S Mk1 Rally Car ‘67 (Gran Turismo 2 Return)
- Mitsubishi Lancer Evolution VI Rally Car ‘99
- Nissan Silvia 240RS (S110) ‘83
- Peugeot 205 Turbo 16 ‘85
- Subaru Impreza Rally Car ‘95
- Subaru Impreza Rally Car ‘99
Le Mans Racers (10 - Returns):
- Audi R8 (Audi Playstation Team ORECA) ‘05
- Audi R18 TDI Prototype ‘11
- Bentley Speed 8 ‘03
- BMW V12 LMR ‘99
- Nissan R390 GT1 ‘98
- Nissan R89C ‘89
- Pescarolo Pescarolo C60 Hybride - Judd ‘05
- Peugeot 905B Evo 1 Bis LM ‘92
- Toyota GT-ONE (TS020) ‘99
- Toyota MINOLTA Toyota 88C-V ‘89
Gran Turismo New Stars (12 - New):
- Aston Martin Vantage ‘20
- Audi R8 V10 5.2 Performance ‘18
- BMW M2 Competition ‘18
- Citroen C3 Aircross ‘21
- Ford Fiesta ST ‘18
- Ford Focus RS ‘09
- Lamborghini Urus ‘19
- Maserati GranTurismo S MC Stradale ‘11
- Mercedes-Benz C63 AMG Coupe Black Series ‘11
- Porsche Cayman 718 GTS ‘19
- Toyota Yaris GRMN ‘19
- Volkswagen Up! GTI ‘20
Gran Turismo Stars (12 - Returns):
- Alfa Romeo Giulia Sprint 1600 GTA ‘65
- BMW M3 GTR ‘03
- Ford Focus RS ‘02
- Lexus LFA ‘10
- Mazda Demio GL-X ‘99
- NISMO 400R ‘96
- Nissan Skyline GTS-R (R31) ‘87
- Pagani Zonda C12S 7.3 ‘02
- RUF CTR2 ‘96
- Suzuki Cappuccino (EA21R) ‘95
- Tommykaira ZZII ‘00
- TVR Cerbera Speed 12 ‘00
It‘s probably overkill (100 cars in total here) but again, it’s an idea which I feel would work fairly well - a mix of everyday and sporty cars with some old favourites thrown in there too.