HUD is something I also was thinking about. Get rid of all the clutter a dashboard brings. And why not have the centre display as a iPad air? Removable so car will only work when screen is mounted. Also good security thing, store some sort of password on iPad en car will only start when it's correct.
May get some people not liking it but I'm sure the majority will love it.
We have people complaining that the current Model S fob is to intrusive as it is. I doubt many people are willing to carry around a tablet as a key.