Classical head musicians would love staff editor, as implemented in Reaper for example.
i think this only affects a tiny minority of users and the effort it takes to implement such an editor is not worth the time at the moment, putting this as lower priority
I think the way this would be implemented is we would support MusicXML internally and have an option to edit it in an external app that supports musicxml
so only the internal MIDI <=> musicxml conversions need to be implemented
Musescore converts well MIDI <=> musicxml, so however nice, musicxml import -for me at least- is not so different from midi import. The ability to visually see the staff with the playhead running over it when playing would be really cool - even if the actual editing of the music is in the usual piano roll system. There is a cool libre project that implements this functionality - Laborejo - https://laborejo.org/laborejo/