A watchOS 2 app that taps you at specified intervals. My old Casio watches have a feature where they beep twice at the top of the hour. This helps me be aware of the time without having to look at my watch. This would be even more useful if it were undetectable by others, via a tap on the wrist. This would also be useful for when giving presentations, so that you are aware of your time, again, without looking at your watch. I suppose this would be useful for any timing application. Right now, you always have to wake up your watch to see the time.
There's an iOS app that basically does this: http://buzzclockapp.com/