There is a nice document in the career design that shows what cars they plan to licence for each simulated series. There are still some series missing a car at all:
Stock car ladder 1, 2, semi pro
Touring ladder 1, 2
Small prototype, sub-Radical SR3
Formula ladder 3, 4, Indy
Then again, they exceeded that document already in some places, the Z4 GT3 wasnt part of that for example or the Aussie V8 Ford.
In my personal opinion (so please dont shoot), I wish they d have focused more on filling the career mode series with the most cars possible and kept the special/historic cars out of the main game. I dont see the sense in the 1940 BMW 328 for the main game for example, they could have made stuff like that, the historic F1s or 1980 touring cars dedicated DLC instead.
Imagine: A historic F1 DLC with the cars and tracks of the 1967, 1978 and 1986 season and a dedicated career mode branch. Or a 1980s DTM/BTCC DLC with the M3, Sierra Cosworth, Audi V8, Merc 190E. How awesome would that be and who in their right mind wouldnt buy that in a heartbeat?!
The current "something for everyone" approach imho takes quite a bit of the impact those things could have away imho.