a killer feature for gps devices would be if the gps lady can sense when a person is having a conversation with someone else, or is listening to a song, so rather than butting in, gps lady should "clear her throat" and wait politely for a minute. Then she can escalate with polite cough, and then to a "excuse me...". Only then if its the last possible minute should she butt in with her stupid directions.
She be easy to implement; all you need is for the gps device to have a microphone to sense the ambient noise level inside the car.