To take Zwift to another level I would like to see a couple of features.
I think many people would welcome a improved custom workout designer. Features I would like to see:
Web-based for easy access and not having to go through the whole login routine.
Export and import functionality (for sharing your custom workouts)
Folders and subfolders
A better design for the description of selected workouts.
Hope this is features that will be added in the near future.
Check on the export import + folders (https://support.zwift.com/hc/en-us/articles/208467596-Using-Workout-Mode