RTC: Damn good idea, so long as the empeg can pick it up and display the time.
This would mean associated discovery and clock management software on the empeg.

One of the input channels of the ADC could do with an electret mike and a suitable pre-amp to be used for speed/ambient noise detection (to allow the playback volume to be jacked up).

Fused outputs would be a good idea, in spite of the semis being more likely to die before the fuses. Alternatively, relay-based outputs would be fine.

The digital I/O should (if at all possible) be opto-isolated.

If the control stalk interface was jumperable to select type, this might be useful. If you include the Renault control stalk i/f, you have at least one customer ;^)

A couple of properly isolated RS232 ports might be fun (scrolling rear window LED displays, etc.). Mind you though, this would need a bit of extra RAM to be much use; any chance of a bit more RAM?

A board design laid out to match a commonly available casing (Farnell? RS?) that could be used for a build-in would help.

Some control software on the empeg might be needed for basic functions, but I suspect you are looking for a hardware only package here - most of the smart cookies should be able to figure out nice things to do with it anyway ;^)