I keep wondering that myself. AOL, Yahoo, and Microsoft all went through this same thing.
What Apple needs to do is not just keep updating components for the market they do have, but keep exploring new markets.
One example that would be not "plug and play", but it would take off would be a car audio head, perhaps with a mechanism similar to the PowerBook Duo of yore for "injecting" and ejecting an iPhone on the go. The unique features it would offer would be Siri, and depending on the vehicle and access to the CANbus, some additional things.
There is always the "iKey" that Apple has gotten patents on, which uses near field communications so that the device can replace a badge at a card reader. Apple could creak its way into the enterprise with something that works with HID readers and a lock screen that shows an employee picture. Combine that with the low-level ARM function of "worlds" that can separate personal use of a phone from work stuff, and one device would work as a universal key, be it for a car (like higher end Nissans or the Prius), door access at work, and even a PIV/CAC reader for higher security stuff.