Personally, I've never been a huge fan of GT soundtrack, so I like the idea of playing the music from my hard drive. Being able to select two different folders for menus and race soundtrack was brilliant!
For GT6, I'd be glad if they sticked to this system; but it would be even greater if the stereo system in the car could display the name of the track being played (just like in real life!). And one more thing, as the music is played randomly from the selected folders, it would be awesome to change track from pause menu.
While in the general menu, photo mode, car dealer, tuning parts shop or car maintenance & service (yeah, I hate that track), the music could be changed via a pop up menu by pressing the triangle button for instance.
To a further extent, the triangle button could be used as a multitask feature (a la i.O.S.) to be able to move from a game mode to another one without being forced to quit everything. Amongst this multitask features, sound menu would appear on a scrolling bar as an icon...
Just some thoughts...