An idea I've toyed with now and then is replacing my car stereo with a computer. The benefits are many and varied. It would be able to store a large music collection for listening on the go and wouldn't need any portable player dock or cables. It could still be made to pick up radio signals, but could also play (or rip) CDs, DVDs and TV. Playing video would, of course, be for passengers only. It could connect to the home network over a wireless link and provide internet access when in range. I'd also incorporate a GPS receiver and navigation software.
It would have to operate via a touch screen on the front, because a keyboard and mouse are ridiculous in this context. It would also have to be very power efficient to run off the car battery. Sound output would be through the existing car speakers, of course.
Mokalus of Borg
PS - Any other ideas may be left in the comments.
PPS - This is, of course, just a dream so far.