I also think this would be a very useful feature.
Tactics Trainer
Tactics Trainer only added the "suggest tag" feature a year or 2 ago. When it was implemented Staff said there were plans to do exactly what you suggested once the problems were accurately tagged. I have no idea of the current status, but someone in charge seems mildly obsessed with the idea of constantly adding new puzzles which can only delay it.
Is it possible to search the Tactics Trainer for specific themes? For example the bar graph below the Tactics Trainer shows I am weak in problems using Zugzwang. I would like to search out all problems using this theme of Zugzwang so I can bone up on it and improve my chess as a result.