I once published a wishlist of what I would like my audio player to do. In short it would allow me to play music wirelessly from my home. That means a large amount of data would be downloaded from the net. And even though it's possible with UMTS, Wifi or WiMax (yet to appear) the cost would be huge. And besides, I couldn't listen to the music I want where the wireless access is unavailable.
So the interim option would be a device in 2 parts. One small part that would have a screen, a Bluetooth 2 (stereo) connection and a few buttons. Lots of current phones would be enough. The second part would be a large HD. Last time I counted 300 or 400 GB would be enough to store all my music losslessly. And such drives already exist in 3"5 format. Even though it would be heavy to carry, it would be almost OK for everyday use. This HD would need an enclosure with Bluetooth connectivity and USB 2 or Firewire to feed it. And of course enough battery to run it. Which might be the main problem as such big drives need a lot of power to work. The Bluetooth software could be almost all handled in the HD, close to the data and the phone would just run a Java applet to access this software (music server).