Improved visual and physical damage.
If you plow into a wall at over 100mph, the bumper and bonnet should crumple, glass should shatter, lights be non existent, wheels buckled. If you roll the car, the roof should crumple, glass shatter etc...
In GT5 you could do practically do anything, and receive nothing but some scratches, scuffs and dents. This damage should also carry over from race to race, will gradually get worse, requiring you to fix it. This is general would encourage better more skilled driving to avoid ruining your car, which would cost £££.
However there should be an option to turn this on or off, and have a scale severity when on. E.g, minor damage (few dents, broken glass) to major damage (wheels buckling, potentially falling off or tyres bursting, crumpled body panels etc...)
Washing the car should make a difference. And the animation when doing this should be a little more updated?
Visual Customisation.
Allow better use of painting cars. Maybe choose individual panels rather than entire car. A wider variety of wheels rather than the select 20ish for the entire game. Also wheel size making a difference to handling, grip and acceleration(not essential but would be nice), depth and diameter(visually). Custom and stock liveries and decals. Would make the game a whole lot more entertaining if you could 'really' change your car you how 'you' want rather than just a new paint job and wheels.
Sound
Just in general needs to be massively improved. A lot of the cars sound exactly the same, especially when sticking the exhausts on. On multiplayer I find that other players 'do' all sound the same, mainly when parked and revving. So a wider variety of engine and exhaust noises, that accustom to the performance changes made. Also sliding, braking, drifting and crashing sounds need to be improved with the same increase in variety.
I feel like if they are going to create a game this big in scale and variety the small things need to match up. Sounds, visuals, and physics etc...