For me, the most obvious is Porsche. EA are assholes for not alloing other developers use Porsche in their games, and they are my favorite car manufacterer. I would specifically like to see the Porsche GT2 RS, Turbo S, GTS, Carerra GT, GT3 RS, and the base 911. I also wouldn't mind seeing the exact same cars for older generations (996 and older) to see how much well a modern base 911 compares to a 10 year old Turbo or GT2.
After that, I would like a few Konigseggs, a few premium Aston Martins, Saleen S7, a Gumpert Apollo, Bughatti Veyron, SSC Ultimate Aero, and most importantly, an Ariel Atom 3 and 500 V8, how do no racing sims have this car? A Ferrarri F430 (not the Scuderia, the base car). Ford Mustang GT (how was the current model 400hp Mustang GT, nor the 300hp V6 not in GT5? This is the standard American Sports car), The Cadillac CTS and CTS V, and the Lotus Exige.
For some old classics, a BMW 8 series, Ferrari F50, all old gen BMW M3's (I think this is the reasonably affordable car most people wish they had), The Corvette C3 ZL1 Stingray, the DeLorean (there is only a remake of it, not the original), and some old 1970's-1990's Benz SL's (also the first AMG one if possible).
For new cars specifically for GT6, I want to see the Lamborghini Sesto Elemento,and the Aventador. The new Porsche 911 (991 gen), the CTS V Coupe, Benz C 63 AMG Black Edition, BMW 1 series M Coupe, Audi RS5, maybe a Bently Continental or Rolls Royse Phantom/Ghost, and well if Top Gear Review it, it should be in the game IMHO.
I don't mind having a boatload of standard 90's civics, but if were going to have a pile of shit cars, I would like to see a lot more outside Japan. I mean I want to see some base BMW 3 series, base benz's, Renaults, Alfa Romeos, Ford Focus's, Golfs, Jetta's, Mustangs, Cameros, and old Cadillacs