I think it would be most excellent if there was more tag information that you could edit such as disk number, album artist, record label, and composer. Also, being able to have multiple sort priorities (For example: If I wanted to sort my files by artist first, album second, disk number third, and track number last).
You can already display and edit those fields. Simply add them to the Tag Panel via Options > Tag Panel or to the file view via View > Columns.... A complete list of all supported fields is available in Mp3tag's help file.
Regarding your sorting suggestion: have you tried to click the column headers in reverse order? Mp3tag uses a stable sort algorithm, so the files should be correctly sorted afterwards.