Apologies in advance if this is a dumb question.

What is Apple's next move once it saturates the market?

Or, more generally, what is the next move once you've saturated a market?

A whole new product line. See GE for an example of a company that has figured out how to just keep growing.

You create new product lines. They've done the Apple Watch, Apple TV, Apple Pay, etc..

There's plenty of more options available in both the short (<5yrs) and long term (>5yrs). They've previously had consumer product lines that included cameras, game consoles, laser printers, TVs, etc.. They could revisit those markets again, or push into business markets.

Really they could do anything.

They are rumored to be interested in cars

Define your market more broadly. The iPhone itself is an example of this. Desktop computers don't have much growth? Well, what else could a computer be besides a desktop?

It’s safe to say that the whole Internet of Things is still in its “feature phone” stage, if not the infancy of that.

a car :)

creating new products and services which will continue to grow or change the markets and get new customers.

Product innovation I would think

You make a new product. But without Steve Jobs it's hard..