One of the reasons I bought an Apple Watch was to be able to set high-heart-rate notifications during workouts (at the time, I was experiencing some odd symptoms, and my cardiologist asked me to keep my HR low). I had been using the WorkOutDoors app, which offers various alerts modalities for lots of different metrics. While it wouldn't be in scope for Superset to try to match WorkOutDoor's configurability, being able to set a few different types of alerts would be appropriate.
A few metics that would be useful to alert on:
  • heart rate (“High heart rate!”)
  • heart rate zone (“Now in zone 2”)
  • time duration (“30 minutes elapsed”)
  • calories (“250 kcal burned”)
  • strain (“50% strain reached”)
  • cardiac load (“50% cardiac load reached”)
  • weight lifted (“2500 pounds lifted”)
The alert can simply be a haptic with a message displayed on the screen for a few seconds.
In case it's helpful, I've attached a few screenshots of WorkOutDoor's alerts configuration screen.