The only things I can think of are aesthetics, capacity, integration, ease-of-use and on-demand.

Aesthetics: A color screen would be nice. Different fascia. These things are largely irrelevant.

Capacity: Really, very few of us have the need for the huge amounts of storage already available, but maybe.

Integration: It would be nice for it to be more closely integrated with the car, making built-in buttons work more easily, deal with information from the car, etc. I think that some sort of data interface with the cars electronics would be neat, rather than having to hack anything in that's not simply audio. Multiplexed data from OBD systems, onboard GPS, fuel, oil, door, button monitors, etc. could be very useful. Output to a HUD would be neat, too, or even to a rear seat interface.

Ease-of-use: Could be stuff from buttons to making music easier to load (transportable media, wireless network, etc.) to UI improvements, possibly based on UI technologies that don't exist yet. I'm not sure that voice commands are the panacea people think, at leat not until it could interpret ``play me some Bosstones'', rather than ``Playlists. B. B. B. Damn it! Left. Left. Left. Left. Damn! Right. Down. Down.''

On-demand: It would be nice if I could find music online without expending so much effort. Of course, I can, really, but I don't feel like paying as much for it as if I went to the record store. I'd like to be able to hear a few songs before buying an album, but if it costs me as much to download the songs as buying the album in the first place, then I've either saved no money or spent twice as much. So it'd be nice if there were programmable internet radios or something like that, where I could tell the empeg what I wanted to hear and it would find it. Preferably while in use -- not as part of some configuration-time activity.