Subbing exercises for similar movements, not every exercise in the library.
When a client hits substitute, they are presented with every exercise in the library. Instead, it would be nice for it to at least be narrowed down to the same muscle group. If a client is scheduled to do a squat, they could be presented with a goblet squat, not pullups/curls/etc...
Mike Zimmer commented
When you're in the exercise library or the workout builder, and you click an exercise to view the video/instructions. Have a section that shows similar and/or recommended exercises to the exercise that you're viewing.
Very similar to how Amazon has similar and recommended products displayed to the one you're viewing.
Ability to specify a list of exercises I prefer to substitute for an exercise.