Great idea! One other thing that annoys me is the inability to trim workouts, mostly walking workouts which I forgot to stop. Even when the Apple Watch asks you if you want to end the workout, it doesn’t cut off the end where nothing happened anymore. It should be possible to do this (semi-)automatically using the heart rate and movement data.
That's a good problem to solve. Another pet peeve of mine is the inability to modify different streams of fitness data in a single workout. If I record workout on the watch, but use a different device just for heart rate, there's no good way to replace the watch recorded HR data with this other device's HR data.