I raised this back in May 2025. You can scroll down through this thread. At that point, based on the response, it felt like it had not really been considered 
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It is crazy how we have the ‘Splits’ feature which works great for Free Ride etc. but we can’t see avg watts per interval when in workout mode.
The customisable HUD coming to workout mode in May ‘26 is promising, but from what has been mentioned, Zwifters will only be able to select from the current 13 options, which does not include avg (lap) watts.
TrainingPeaks Virtual has this feature perfectly integrated. I can see my Avg Power, HR and Cadence per interval along with a host of other metrics.