Some time ago I asked about chapter support in Musepack files and then you suggested to display them as "virtual tracks".
I think this is a brilliant idea, because it can be applied to (almost) all file types.
Currently I use Musepack chapters for conceptual and live albums that usually have subtracks.
Both players that I use (Foobar and MusicBee) works fine (with some slight difference) with this native feature of the format.
However, editing is almost impossible, once the chapters are embedded during encoding.
I wanted to try mp4 chapters support, however I still cannot find a proper way to include chapters to mp4 (m4a).
About ogg, although it seems quite easy to make the chapter tags (ogg chapter extension), there is no player that supports them except VLC (partly, only display chapters, but cannot jump to a subtrack).
Loseless formats (flac, maybe others) can have embedded cue sheets, but again, inline editing is hard or non trivial.
So, I still that it would be a very useful extension if mp3tag can support natively display and editing of chapters and cue sheets as "virtual tracks".