Normal car mechanisms
It would be cool if there was a clutch in the car would be awesome + gears. Obviously you could have an automatic gearbox if you cant shift but it would be slower. Some cars could even have a CVT transmission! With this mechanism you could go faster and accelerate faster from corners. With the gears in mind if you could also change the gear ratio of the car (make the gears longer or shorter) you could make your car accelerate faster or have a better top speed.Better suspension would be nice especially for all the offroad cars. If you could tune the suspension you could make it soft for softer driving and better of roading or make it stiff for better turning on the road and faster acceleration
Car mechanisms for fun
I was thinking for more fun what if the servers cars had some kind of drifting mecanism. So RWD or AWD cars could drift and maybe FWD with little use of handbrake.
Drifting would make the driving a lot more realistic and you could make some kind of drifting events in open areas like parking lots.
I know that you can drift now but the drifts last for like 1 second and then you have full grip again.
Bonus
Also if you could add some kind of ATV to the vehicles it would be really awesome !
I know this is pretty hard to make but just trowing the idea out there !