[microtherion]

I've written two music apps that use MusicXML as their native representation (https://woodshed.in is the newer one), so I've been involved in this world as well.

MusicXML is a great effort to tackle a very difficult problem, but some of the details can get rather hairy (e.g. having to represent many concepts twice, once for the visual aspect, and once for the performance aspect; or how exactly to express incomplete slurs). Interoperability in practice seems to be fairly limited (Possibly because for many music programs, MusicXML import/export is an afterthought).

One of the biggest contributions a new standard could make is to provide as complete a test suite as possible of various musical concepts (and their corner cases!) and their canonical representation. It looks like MNX has already made good efforts in this direction.