Apple's (NASDAQ:AAPL) long-awaited deal with China Mobile (NYSE:CHL) finally arrived -- an early Christmas gift from both of these heavyweights. China Mobile wins, as it can regain users that it has lost to competitors such as China Unicom that have been offering the iPhone for quite a while now. Apple seems to win because this broadens its reach in China -- a market that it hasn't quite taken by storm. While this deal is nice, the real question that Apple investors should be asking is, "what's next?"

A "big" iPhone is the next step
Apple already has a good chunk of the high-end handset market to itself, particularly in developed nations. Its brand is powerful, its products superb, and its customer base loyal. Unfortunately, it's tough to ignore that Apple really is missing out on a good chunk of the high-end market by not offering a larger iPhone. This isn't to say that Apple needs a 5.7-inch or 6-inch monstrosity, but a 4.5-inch or 4.7-inch device could go a long way in snatching back some of the share that it has lost at the high end to Samsung (NASDAQOTH:SSNLF) and perhaps LG.

While trying to predict consumer buying patterns is anything but easy, it will be very interesting to see just how a "big" iPhone -- perhaps the iPhone 6 -- could perform in the marketplace. Apple could conceivably continue to offer a 4-inch device as an "iPhone mini" and then the larger 4.5-inch or 4.7-inch device as an iPhone Air. The smaller iPhone could still go for $199 subsidized for the 16GB model, while the larger one -- which would probably come with more memory, a higher-resolution screen, and other extras -- could probably fetch $299 for the base mode. That would mean a bump in iPhone ASPs -- and share gains, to boot. Win!

But what's after that?
Eventually the high end of the smartphone market will saturate, if it hasn't already. At some point there will be no more share left to take and plenty of share to lose. Apple will need something new to drive the next leg of growth. It's difficult to use the word "innovation" here because all of Apple's products -- as incremental as some may seem -- have had plenty of innovation, whether it's on the hardware side or on the software side. A more apt description of what Apple needs is an expanded total addressable market.

Some speculate it's an iWatch, and others expect an iTV. But really, these are almost too obvious. Samsung already has a smart watch out, although it has been widely panned, and smart TVs with rather sophisticated UIs and processors are already fairly commonplace -- just ask Samsung.

Apple's going to have to revolutionize -- as bombastic as this term is -- an industry that people didn't know needed revolutionizing. What will that be, and will Apple really strike gold again? Only time will tell.

Foolish bottom line
Apple is a great company, but the real problem is that it's showing signs of maturity as the industries it operates in -- high-end handsets and tablets -- mature. This maturity will hit its competitors in the lower end a bit later than it'll hit Apple. But it will hit them. If Apple can successfully reinvent another industry or invent a completely new one, then its growth story still has more legs. If not, then Apple has matured and its shares don't look all that compelling today.

Ashraf Eassa has no position in any stocks mentioned. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple and China Mobile. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.