Just thinking about that custom car holder... What if the "other half" for car was actually whole unit plugged in place of car audio receiver, containing holder for the phone, small amplifier, FM-radio and connections to other car functions like remote controls or even ODB-port. Plugging in the device would change the UI of the phone so that it would automatically become main interface for radio, audio, GPS/navigation etc. functions, automatically connect to hands free system in the car and charge the device at the same time so there wouldn't be any need for separate charging cables etc.