One aspect I really wanted, and which is already available but could be improved and would significantly improve the quality of the gameplay experience, is the camera setup and adjustments. This is what most impacts my gaming experience compared to other games like the Forza series, Gran Turismo Sport, and even the first three in the GT series.
Being able to adjust the chase cam beyond what we have today with a FoV option, a wider range of distance, tilt, roll axis sensitivity, and perhaps other eye candy like camera shake.
Default
wide FOV + higher tilt and closer
Adjusting the FoV of the cockpit camera, a wider range of seat height and distance adjustments, an option to remove steering wheel and driver animations for those playing with a steering wheel, a hood camera with a rearview mirror, and, in fact, an option to have a virtual rearview mirror on any camera.
Hood camera
Another interesting thing would be to expand the audio customization options in addition to the options we already have for adjusting tire volume, gearbox, etc, for example, you could adjust an equalizer for all engine sound frequencies, increasing the presence or volume of the engine and exhaust individually, and perhaps adding reverb effects.
Additionally, you could customize which songs you want to use in each menu, with the option to use your own songs from the USB drive.
Simple and common things that, IMO, would greatly improve the gaming experience and enjoyment.