I took this from the WWDC app. It was updated a few days ago. As you can see, there is a new category called watchOS. Apple might decide to give the Apple Watch an operating system of its own.
hmm i mean isnt watchOS currently based on iOS?... and developers need WatchKit to write apps for it. What if Apple decided to write a new OS from scratch? and we have to learn a different way to write native Watch apps. This could be exciting.
All of the iOS flavors are based on OS X. So basically every Apple OS is a cousin. But each OS has its different uses and APIs since they're modeled on different devices. You're not going to have Digital Crown APIs on anything but Watch OS. Watch OS also isn't going to support quite as much multitasking as an iPhone, especially not an iPad.