This is a brilliant plan! The biggest killer of Mac Front ends is dev's who loose interest half-way through. Being open source this shouldnt be nearly as much of a problem.
Some things I have not seen mentioned:
Growl Support! (
http://www.growl.info)
Sparkle for auto-updating (
http://sparkle.andymatuschak.org/)
Bluetooth Proximity detection (
http://www.technocrat.ca/?p=44)
Built in widget engine (either Apple's widgets or Yahoo's)
Like everyone else I want ODBII, Nav, and VLC/Mplayer support but lets start out with some easy stuff before we tackle Everest.
As for the interface, I was thinking that an iPhone-like interface would be very cool and easy for in-car-use. Simple aqua squares with each function (or widget) on a grid. Move squares around or even remove the ones you dont use. Just my 2¢.
-KRS Juan-