After reading a bit through this thread, it seems people like to focus too much on some things unrelated to the racing experience, and others like arcade racer functions. Remember, this is supposed to be a sim. In real life, you can't "save and store" multiple suspension setups.
Here are a few realistic things I would love to see:
-Ability to chose your tire and wheel size, width and height, along with offset and have these settings affect the actual performance of the car. In some cases, this can have a bigger performance gain or decrease than suspension mods.
-More tracks, specifically some well known tracks from around the world besides the best known. there are some tracks that are far more fun to drive, but are less known, in the world. For example, Mosport International Raceway in Ontario, Canada. Turn 2 on this track is rated among the top 10 hardest to master in the world and used to host a number of different forms of cars, from CanAm monsters to at one time F1 type cars. It is currently a stop on the ALMS circuit.
-Begone with some of the multiples of some cars, especially the slowest cars that are utterly useless (Midget D-Type comes to mind). Replace these cars with a greater variety of everyday cars that many people drive, and not just in Japan. There are cars like the Dodge Neon that are very popular with autocross racers. Why is it that I can't buy ANY of the vehicles that I have owned in real life in the game?
Thats my take on things anyway. Sure there are many other things, but these would mean the most to me, but I wouldn't be hurt if they didn't come.