Some suggestions for new things
1- Open a smattering of valves such as pressure springs and diameter of valves because they could earn some Hps and also some revolutions to solve more and valves fluctuation problems
2 Limit RPM for more than 20000rpm, for even that is not so usual in road cars or even sports is an additional option that makes the most real and cool game.
3- Add the alcohol (ethanol) as fuel as well as opening more engines possibilities also in campaign mode would be something cool because some regions like here in my country (Brazil) use quite as fuel mainly in bi-fuel vehicles
4 adjustable suspension, a system that has in some sports or luxury cars that can switch in sport or normal mode, could reflect on the viability and in the question sportiness and comfort in getting good play on both sides.
Those are my suggestions ... sorry if you have any English errors.