This may be a small thing (and could even be added to 2.0 with a patch 😉😉) but in custom championships I think you should be able to choose specific cars in a class, not just the whole thing. The Historic championships have this already. For instance, in the first championships, there's an H3 RWD series featuring the 240Z and the Stratos. Then in the 80's ones, there's a series with the rest of that class' cars.
Something like this in custom championships would be really nice (for me, at least). Makes it easier to create a more authentic series.
EDIT: Another I thing I just thought of (sorry if this has been mentioned before) is that the specific gearbox/transmission (might be misusing the term) and existence of racing handbrake should be authentic to the car. For example there are plenty of cars in the game that don't/didn't have handbrakes you could actually use while driving, but you can handbrake in every one of them. Another example is that the Mitsubishi Lancer R4 (I read this somewhere) has a dogbox manual, but in the game it's a traditional manual.