I'd like to be able to change the tyre pressure, among other things but this esspecially. Its annoying that you can change the camber angle of the tyres but not adjust the pressure acording to the angle, therfor making the camber option a total waste.
Flags at tracks, such as a blue flag when overtaking, and a yellow if there is a danger to other drivers on the track somewhere, blumin' annoying when you charge around a blind corner and come face to face with a plank who can't point the car forward ... etc, etc.
Choice of tyre/wheel width.
Ablity to flip the car over (realisticly as possible).
Concrete that isn't mascarading as a tyre wall. If a tyre wall is hit i'd like to bounce and see the wounded wall (same with all other materials).
Grass and the like that actually dirties the car during races, giving a valid point to having a car wash feature.
And of course brilliant AI (as if its not obvious).
When doing a company such as BMW, Aston Martin or other such prestigious manufacturers then at least feature some (i mean all really

) of their great work, rather than a handful, and not waste time making a steam carage from the 18-bloody-hundreds!
Heritage tracks, Donnington, Silverstone, Oulton Park (thats pretty biast due to my nationality, but hey, sue me!) And certainly more Locations (Australia, South Africa, China .. etc.)
Real championships (WTCC partularily

), although there is a problem with their monopolisation.
Being able to decide your own time of racing, would love to warm my rubber at the 'ring during sunset, lets hope they make my dream come true.
If a player of the GT series has completed their license tests in any other game (maybe restrict it ot just GT3/4) then they should not have their intelligence patronised by forcing them to take a wide birth corner in an S2000 following a big blue line ... give us a break! 👎
Real currency.
The ability to upload your own music from devices such as MP3 players and devise your own soundtrack for the game (if you wanted to that is).
No damage - unless it's incredibly realistic, which i doubt.
Proper rally areas around europe.
Some public road tracks, what springs to my mind is the Isle of man TT course, or famous car driving areas of scotland. (and no yankee tracks, roads are all 90 degree's and pretty damn boring, in reference to the GT4 New York track)
The ability to save replays and upload them to the PC, its possible with GT4, but i don't know how, and i'd like a straight forward option to do so.