(not about plug-ins but advanced enough functionality and therefore not for the main thread either)
I applaud the recent regex function added to tag editor. Although I have not tried it yet, it would simplify lots of tag manipulation functions I wrote in the past and I plan to write in future.
I recently noticed that Playlists can be added to the views as columns. I found this very useful. I wonder if the same field can also be accessed in calculated tags - I could not figure out how. This would have very powerful applications in analyzing playing history of a song. For example, when I DJ, I keep the playlists of events in a standard format that includes date and event name. Ability to access list of playlists would allow me to create tags like:
"the number of times I played this song in a particular venue" etc.
This would be useful DJing tool and I would be able to brag to other DJs about how cool MC is.