Proving Grounds Track
Basically you would have a big oval track, like the Forza Benchmark track which is 4 miles long. It wouldn't need to be that long, but long and with high banking so cars can run flat out. Also have a drag strip section, skidpad, suspension testing, cornering ect.
In Game Engine Modification
A feature where you can install a custom engine. Sliders allow you to adjust torque and hp, add super/turbo chargers and other mods.
Edit: One more suggestion....
Proving Grounds and in game Engine Modification
Basically you would have a big oval track, like the Forza Benchmark track which is 4 miles long. It wouldn't need to be that long, but long and with high banking so cars can run flat out. Also have a drag strip section, skidpad, suspension testing, cornering ect.
In Game Engine Modification
A feature where you can install a custom engine. Sliders allow you to adjust torque and hp, add super/turbo chargers and other mods.
Edit: One more suggestion....
Proving Grounds and in game Engine Modification