Maybe some supprt for an ODB-II interface hardware (bundle drivers for the popular serial and/or usb cables).
I might be stating the obvious as well:
Fast boot time (maybe using suspend?)
Small amount of required disk space
Easy installation via touchscreen (i.e. automatic required kernel module detection)
Easy way to change settings via touchscreen (could maybe be achieved by frontend)
navigation support and all that, but what happens if different front ends use different libraries. Could having unused libraries significantly increase the overall size of the OS and is that of significant concern? Could the end size of the OS be reduced by an installation program that only installed required drivers and libraries or would such an installation program be too difficult to program?