MacBooks (the whole lineup) use glass for their trackpads. Why not sapphire from their new plants. Why use sapphire? Because it enables TouchId (okay, I don’t know that you must use sapphire for TouchID). What else do you need for TouchID? The secure enclave on the A7. Instead of pulling out the secure enclave into a separate chip, why not just put in an A7. It takes more than a few leaps, but I’m interested in brainstorming right now.
Update: If I’m going to speculate, why not put it in every Mac? How the user interacts with it is a design question, but from a technology perspective it would great to have TouchID on all machines. The economies of scale bring the cost of the A7 down even further.