Not a complete list..
UI redesign (functionality, not aesthetics) - let me access my garage no matter where in the game I am, make all transitions faster than before..etc
Improve AI behaviour - more unpredictability in their actions, better intelligence overtaking
More diverse lineup - you touched on this but not in exactly the way I want. I want all those weird little street cars and JP cars to remain. But the hundreds of new cars they add to the roster need to have a greater western focus, because they're missing too many classics. It would also be amazing if they could add a proper F1 roster to the game. (cut the go karts and nascar if you have to...)
Better online integration - both in social - in sharing your car designs, replays and photomode stuff with others - and in racing - in customizing race conditions, setting up tournaments with friends etc.
Completely overhaul the car customization system. Fuck paintchips.
Time and Weather transitions on all tracks.
Physics - because there's always room for improvement.
And do something to A Spec..just..make it better.
