I would very much like different dedicated tuning shops as well as smaller special part stores so you have to find the stuff you need to create your perfect ride. The amount of customizing should be so huge, that no car would look alike.
And if possible, cooperate with real world tuners.
Purpose tuning like Circuit, Rallye, Street and Pro-Street, Trail, Drift or Stunt would be cool.
And i would also introduce multiple racing leagues you can enter.