Particularly needed, from my perspective, would be for example SYLT (=lyrics with timecodes, which can scroll down as you sing along or even be displayed karaoke-style). Feature request and reasons here.
Also useful is UFID. That's where a song's unique ID can be stored, such as a MusicBrainz ID retrieved through acoustic fingerprinting. UFID support in MP3Tag would allow adding a file ID column, showing the user which files already have and which still need identification.