Cars. Absolutely no doubt they need to be in that industry.

Cars are pretty firmly in iPhone slow growth territory. If anything we expect to have less of them in the future.

That, plus car manufacturing is a 10% margin business, Apple is closer to 30%. There's not a ton of money to be made, particularly because the growth in the car industry that will exist will mostly be in getting e.g. Indians to drive, that'll have even lower margins and isn't Apple's target group.

Self driving cars will be high growth.

How would Apple improve ease of use or add high tech functionality?

Self driving luxury cars. Apple is second to none in focusing on design.

Second to no tech companies, sure, but Apple would presumably be competing with companies like Volkswagen, BMW, and Mercedes which are almost definitely better at designing luxury cars than Apple.

My wager would be AR. We have a few years yet before we see useful, comfortable, well-designed AR environments, but once they cross a certain threshold they'll immediately obsolete a massive section of the computer dislay market, as well as provide numerous opportunities for further innovation. Given that it's a market in which products will succeed or fail based primarily on UX, ease of use, and quality of design, each a strong point of Apple, they have the potential to take the sector by storm.