Apple has been working on modular smart bands for the Apple Watch, according to patent applications surfaced by Apple Insider. The Apple Watch features what has been referred to as a diagnostic port on the underside of its casing, which can power any potential smart band that Apple eventually releases.
The "modular functional band links for wearable devices" patent application hints that Apple could offer a number of different smart bands for the Apple Watch, with everything from extra battery life to blood pressure sensors and even processors. Theoretically this could allow Apple to continuously improve features on older editions of the Apple Watch as it releases new versions of the device in the future.
The idea of smart bands has intrigued several traditional watchmakers I've spoken with as a way to bring smartwatch features to traditional timepieces without having to alter the casing of the watch. Although it's only a patent application, if Apple, and presumably other smartwatch makers, consider building smart bands, we could be in for an accessory explosion in the watch market in the near future.