In terms of frontiers left for Apple to conquer, the living room and the car are probably the only ones where Apple is not competing seriously yet. They have made big progress in the office, in the home (outside of living room), even in hospitals, etc. The AppleTV initiative (either the current one, or the rumored TV panel) will eventually be a serious contender for the living room. I can see the Car as the only frontier remaining for Apple.
Already people use iPods for Car audio - but that only symbolizes everything that is wrong with Car audio today - and is not the solution. I expect a future Apple solution to be a miniature iPad - about 7"-8" screen, that will replace the entire Car Audio unit. Not only will this serve as the Car audio unit, it will also serve as an integrated Navigation System, Bluetooth Car Phone system, Car Information system showing speed, mileage, distance to empty, and even tips to optimize driving. All of this already exists in a variety of apps, Apple will just need to integrate it together and come up with a much better User Interface - which is their USP anyway. I also expect some other Media solutions via apps - like Video Streaming, etc. All this will be fully integrated so that no interaction is possible on the screen directly when the car is moving - but it can still be controlled via a remote controller from the back seat.
The market is obviously massive - and no one has tapped it in a decent way yet. Ripe pickings for Apple.
Regarding their living room initiatives, at some point I expect Apple to release some sort of API that other companies can support - which will allow iDevices to control other appliances in your home. That way you can easily control your lights, curtains, refrigerator, etc. from your iDevices - both from home, as well as remotely.