I'd love to have a 3 layer genre mapping inside of MusicBee too.
What do you suggest though - to have an additional dedicated sub-genre metadata field ?
As you, and others already mentioned before, a lot of what is requested on this matter is already doable by means of custom tags, virtual tags, editing xml's, and other customization.
Yet, that is only reserved for rather experienced and savvy MusicBee users.
The majority of users will not (be able to) use this.
Also, even if you have it clear and laid out for yourself what genres and sub-genres you want to use for your library (for example a list), it is a p.i.t.a. to look up such a list every time you want to enter a genre, sub-genre or style that is not yet present in MusicBee's database.
It would be very nice if this all would be a lot easier, and available to much more users.
My hopes and objectives would roughly be threefold:
1. MusicBee having a comprehensive and actual list of the most common and sensible genres, sub-genres and styles integrated and available.
2. Having the option for users to easily 'unleash' (activate) the possibility to be able to use refined sub-genre and style features within MusicBee.
3. Making it easy to edit and apply genres, sub-genres and styles.
Some four years ago (with some contributions from other members) I created a comprehensive list of genre categories, genres, sub-genres and styles (or keywords).
That list is still available, either to be integrated in MusicBee, or maybe having a new feature in MusicBee where you can insert or edit such a list yourself.
(similar to the existing feature to edit/paste your own list for genre categories.
Many users will not be interested (or willing and able) to explore and use sub-genres and styles.
Therefor it would probably be best to have it optional to choose such an 'advanced genres' mode.
If 1. and 2. are implemented, it would be possible to for example have a dedicated genre/style edit panel.
Maybe something like this, where you have pre-populated drop-down menus, where the next column only shows the options depending on the selection and relation to the column before it.
The biggest challenge would probably be how this would translate to writing and reading tags.
Mapping to custom or existing tags? The use of delimiters within tags to differentiate between genres and sub-genres?
That will probably be some headache material.