Hm, a big oversight, I now see 'artist sort' and 'album artist sort' are already available.
But in drop-down menu's they are to be found under the 's' ('sort artist', 'sort album artist')
Understandable, but for consistency it might be better to have them there under the 'a' ('artist sort' and 'album artist sort').
I ran through all the common tagging fields I am aware of, and made a list of tags that are currently only available by using the custom tag feature.
I divided them in three groups. The first group is what I would really like to see directly available without needing (sacrificing) custom tags.
The second group would be less important to me personally to have directly available, and the third even less.
composer sort order TSOC
subtitle (useful for classical) TIT3
set subtitle (useful for classical) TSST
original album (perhaps useful for classical; work?) TOAL
original filename (useful for classical) TOAF
musician credit list TMCL
album sort order TSOA
involved people list TIPL
initial key TKEY
media type TMED
original artist (performer) TOPE
file type TFLT
original lyricist TOLY
file owner TOWN
net radio station TRSN
net radio owner TRSO
international standard recording code ISRC
commercial info WCOM
official audio file url WOAF
official artist url WOAR
radio page url WORS
publisher url WPUB