First one is to have the volume controls work if you keep the button pressed.
This can be done by setting a timer for the onclick or possibly the mouse down event that as the timer loops it increases or decreases the volume. Then stop the timer when the click event, or mouse down event ends. This will allow for making larger changes in volume without having to keep pressing the vol + or vol - keys.
Second one is how about a clock in the area where the music controls are that is visible throughout. For me the car computer is my only means of looking at time in the car, and this would be what is on a standard head unit.
Third one is set the map objects text property a little larger street names aren't that readable from the drivers seat. Or offer the different sizes as settings.
Then last is to have a key that jumps to the next "CD" so to speak. Where you have the Next and Prev button, if there is more than one CD allow skipping to the next one. Again this is similar functionality to what a head unit would allow. So if you load an album from your library then load another, these buttons would move forward and back through the albums.
Again this software is great, and hopefully these comments help with ideas. As I said in the other thread, this is a great interface and if you want any help and don't mind it being in VB as opposed to C let me know I'd be happy to help.