Some divisions from previous games and groupings based on similar performance/body type (of the cars already announced:)
Early Racing
1948 Ferrari 166MM Barchetta, 1953 Maserati A6GCS/53 PininFarina Berlinetta
Vintage Touring: 1954-1967
Aston Martin DB5, Ferrari Dino 246, Ferrari 250 GT & Cali, Jaguar E Type & XK120, Mercedes 280
1969 Jaguar MKII?
Historic Sportscar Racing: 1956-1968
33 Stradale, 365, 330 P4, 250 TR, GT40, D-Type, Miura, Tipo 61, 300 S, Cobra Daytona and 427
American classics: 1949-1958
Chevy Bel Air, Mercury Coupe, Plymouth Fury
Historic American Muscle: 1964-1973
Javelin, Rebel, Stingray and ZR-1, Chevelle, El Camino, Nova, Impala, Challenger, Charger, Falcon XB and XY, Boss 302, 442, Cuda, Firebird 455, GTO Judge
Classic Mini/Compact: 1965-1980
Abarth 595, Fiat 131, Giulia Sprint GTA, 2002 Turbo, 510, 2000 Roadster, 124 Sport Spider, X1/9, RS1800, RS3100, Cortina, Torana, Mini Cooper, Cosmo, Kadett, Celica
1970s Rally
RS1600, Lancia Stratos, Renault Alpine A110
Later American Muscle: 1977-1979
Camaro, Mustang II King Cobra, Firebird
Hot Hatch: 1980-2014
Abarth 500, Fiesta, Escort, Delta Evo, Civic, Mazdaspeed3, Clio Williams, 5 Turbo
Compact Sport Coupe: 1985-2013
Milano QV, MX5, 240SX, S15, BRZ, Legacy, GT86, Celica ST185/ST205, Sprinter Trueno
Group B Rally: 1980-1986
Audi Sport quattro, 037 Stradale, Delta S4, RS200
Modern Sport Legends: 1981-2005
Vantage '77 and '98, Audi RS2, M1/M3/M5, Regal GNX, Camaro, Corvette, Monte Carlo, Viper, Maranello, F355, Testarossa, SVT Cobra, Sierra Cosworth, NSX-R, XJ-S, 190E, R34, Supra RZ, Carlton, 850R
Birth of the Supercar: 1987-2004
EB110, Enzo, F40, F50, Diablo, F1, F1 GT, R390, S7
Japanese Street Kings: 1992-2009
Integra, S2000, Galant, 350Z, SVX
Rally Day Heroes: 1998-2008
Lancer Evo, Impreza 22B, WRX
Ultimate Sport Compact: 2005+
M235, Z4, 1 Series, Focus, Genesis, Elise, A45, Mini JCW, 370Z, Megane, Astra
Executive Heavyweights: 2011+
Bentley Continental, M5, CTS-V Coupe, Q50, Q60, XFR-S, Ghibli, E63, Wraith, Model S
Ultimate Sport Machines: 2006+
Alfa 8C, Audi RS 4 and 5, BMW M3 and M4, CTS-V, Camaro, 300, Charger, Challenger, California T, Shelby GT350R, IPL G, F-Type, RC F, IS, GS, Evora, C63, S60
Falcon GT F 351, XR8, FPV Pursuit Ute, HSV GTS, HSV Maloo?
Sport GT: 2005+
Z06, ZR1, Viper, Mercedes GT S, GT-R, Sagaris
Exotic GT: 2005+
Vantage, Vanquish, R8 V10, 430, 458, 488, Ford GT, Gallardo, Huracan, LFA, SLS AMG
Ultimate GT: 2008+
One-77, FF, F12tdf, Aventador, Centenario, Reventon, SL65
Track Toys
Alfa 4C, Mono, Caterham, Donkervoort, KTM, Exige, 2-Eleven
Modern Hypercar: 2009-2013
Veyron, LaFerrari, Venom, Veneno, P1, Huayra, Zonda Cinque
Recent Track/Hypercar
Atom 500, FXX K, One:1, Regera, Ultima Evolution 1010
Offroad: 1945-1997
Bronco, Grand Wagoneer, Willys MB, Defender, Manx, FJ40
Street Utility: 1951-2011
Ford Transit, F-100, Syclone, Vandura, FX Ute, Sandman panel van, H1 Alpha, LM 002
Sport Utility: 2011+
BMW X5 and X6, Escalade, F-150 Raptor, Grand Cherokee, Wrangler Rubicon, Range Rover, G65, Ram Runner
Sport Offroad: 2012+
Ariel Nomad, Bowler EXR, Urus, Rally Fighter, Polaris
Extreme/Racing Offroad: 2014+
Ford Ranger T6, Mini X-Raid, Baldwin Trophy Truck, Raptor Race Truck, Penhall Cholla
GT Racing: 2014+
All-Stars Pack: R8 LMS, Z4, C7.R, Viper, 458, GTLM, Falcon FG X, Commodore, Huracan, GT-R