Apple introduced the Digital Crown with the Apple Watch as a means of controlling the device that features quite a small touchscreen. This Digital Crown could make its way to other Apple devices in the future.
A patent found by Patently Apple indicates that the Digital Crown could feature on the iPad or iPhone. The patent says that this control scheme could be used to adjust the volume, resize text, take a picture, or lock the touchscreen.
Both the iPad and iPhone already have perfectly acceptable solutions for these functions, so it may not happen.
One other interesting note in the patent is the mention of devices with no border region; instead, the entire front of the device would be taken up by a touchscreen. This is a possibility, as there have been rumours of late that Apple plans to ditch the bezel and embed the Touch ID fingerprint scanner into the screen.