Here's my million dollar car idea:

Digital maintenance logs

Seriously, how useful would that be? A simple onboard SQLite database (or even text file) you can add stuff to via a phone app. That way, when someone asks "when was the last time this car had an oil change?" you can simply check the logs. You can check when your wiper blades were last replaced, etc. If it's standard enough, Jiffy Lube can update it with whatever work they did.

For now I just settle for a paper logbook in the glove compartment...