First of all, I love this program - one of the most useful and frequently used tools I own
Second, a feature which would be helpful and I imagine simple to implement: Clipboard -> Tag. Basically exactly the same as Text File -> Tag, but use the clipboard rather than a text file
I use the program for tagging a lot of live shows shared over Bittorrent, which for some reason hardly anybody bothers to tag or provide descriptive filenames. Usually there is a text file which includes the set list + other info. Currently I have to edit the text file, save it as a new tag file, then navigate to it in mp3tag to import. If the clipboard could be used as a source, I would just have to select the appropriate part of the file and import directly.
It would save me some time anyway