This feature would be extremely useful for when we're not doing straight up sprint intervals or repeating the same action over and over again.
For example, a strength training workout with a 3x repeat of
* Pushups-25 Reps-Until lap press
* Rest 60 seconds
Then goes to next 3x repeat
* Pullups-10 Reps-Until lap press
* Rest 60 seconds
This would be an extremely valuable addition to your software and I think should be fairly easy on the development side.