Cam Tuning, so you can not only improve the power, but change
where the power gains are in regards to RPM levels). This, when worked with the current turbo system of low, medium, and high RPM turbos, could really benefit tuners that know what they're doing. To make it easy on those that don't, they could have low, medium, and high presets.
Some kind of
electric motor upgrade for hybrid and electric cars. I would imagine this could be done by making some components out of superconductors and rare earth magnets.
Fuel selection would be nice, and would allow for races that only allow alternative fuels/
biofuels. This would also have the more realistic effect of forcing you to use a higher-grade fuel when you increase compression (done in both stage 2 and 3 engine tuning upgrades in GT5).
A variety of oils. I imagine that when you change the oil in GT5, you're putting in some kind of racing oil that improves power. It would be nice to be able to change the oil back to stock, or put in some longer-lasting oil for endurance races.
Launch control, so you don't have to add stability control to keep your wheels from spinning at the start of some races.
The ability to disconnect a set of wheels from the drivetrain in a four wheel drive car.
The ability to buy a differential without any limitied-slip characteristics (which is stock on some cars, so the programming is already there).
Brake disk upgrades, and brake fade on longer races.
More aerodynamic upgrades, including an
underbody, more spoiler options which have different effects (like long, low wings which some drag cars use to get a lot of downforce leverage), splitters (which would allow adjusting the front downforce), hoods with either intake scoops (ram air) or exit vents (downforce, possibly additional cooling), and mirror options (small carbon fiber racing ones, or complete removal).
Tire size options would be nice; not just width, but diameter, rim size (that actually affects performance), and front/rear differences (which the people that drag race might really like, because they could really make their vehicles look like drag racing cars.
It would also be nice if they limited/changed some options in certain cars; I noticed the McLaren MP4-12C had adjustable anti-roll bars, which would be a step down from its self-leveling system. It would be cool if that car's brake-steering system's intensity could be adjusted, too. (That car is just one example, I'm sure there are some other special features on other cars, like the four wheel steering on some. Active Yaw Control is unique to Mitsubishi, but it's available; it shouldn't be a single-case thing.)