Some things that would be good to think about.
1) The ability to balance the "weight" correctly around the tires of the car. Except it will add some minds to overdrive in order to balance the tire weights in order to have extra stability in the steering section, it also will help in the correct consumption of fuel by the engine if you want an economy and environmentally friendly car. Also the animation of the weight correction machine (I don't remember the name of the machine sorry) will be sick

2) More customization of the headlights and taillights. I would like to see more customization over the light section of the car, beyond the 4 arrows, I know it's extremely difficult to program something like that, but it would be pretty neet.
3) The ability to ran the game windowed or at least to fix the issue of alt-tab the game, because it's crashing every time.
4) Stickers for the type of Turbo or Supercharger you use, like TSI or FSI with the red letters indicating how much you have, or V12 Twin Turbo. That would be awesome.
5) Suspension type, like racing or comfort which would make a huge impact in the statistics in the end, like how the car reacts in the corners and different types of roads and if the car would be more racing like or comfort for all family in any road. Also height change suspensions like the old Renaults (we had something like that and was funny).
6) Depending of the overall car manufacturing the company will be more for comfort cars for the family, luxurious cars for businesses and high tier VIP's or the first choice of racing enthusiasts, like the Nissan, Mercedes or Subaru is today.
7) A wind tunnel that indicates the air flow around the car and the statistics be interactive with the choices you make on the fly, like if you put an air vent it will change the entire air flow, again I know it's a headache to program but it will worth it.
That's the things that came to mind when I first played the game and I would like to see. As far as I know the game is awesome and the 35 dollars that I spend worth it till the last penny. You are awesome guys, keep the hard work !