I don’t think it’s as different as you think.
Smart phones are a modular platform already. You just add on to the existing phone (which is an optimized form factor containing all of the “essentials”). You can take an iPhone and stick on a bar code scanner, credit card reader, customize the appearance, customize the ruggedization. Heck, even military applications exist for turning the iPhone into a high end piece of communications equipment for the Army. You have bluetooth accessories like Fit bit, Pebble, etc that extend the functionality without any physical tethering.
Most of other applications you mention are really all able to be driven purely by software. For example remote start and diagnostics can all be done already - check out Torque or Viper Smart Start. My BMW includes it’s own app and many other manufacturers do as well. Mercedes has gone as far as developing a Pebble app.
The beauty of bluetooth and software extensions is the amount of modifications to the phone is zero, and for everything else there’s a USB port on the bottom for a good reason.