It would be great to specify a different intensity unit for each step of a workout.
For example, for my runs I would like to use "% of threshold HR" for worm up and cool down and use "% of threshold pace" for the intervals.

This is particularly annoying when you have a long warm up before an intervals session. You want to use "% of threshold pace" for the intervals (HR won't work because of the lag), but the same intensity unit is used for the warm up as well, resulting in the watch continuously beeping if not in range which is quite possible if not running on pancake flat routes.
Also, running a step below target (slower pace because going uphill for example) will result in a lower TSS

Garmin Connect already supports this and what I am doing now is manually duplicating most workouts.