TL;DR: If you've played Maplestory2 or Mabinogi (or Archeage and some other titles that use it, even a simple minecraft mod that introduces it- that music system but for Hafen
Rather than using the currently key-to-note system that's a slight bit difficult to make songs with, instruments should be able to play songs from a 'song' script (made with parchment of course). Song scripts should support a base melody and a bunch of extra harmonies (Capped at a flat rate, or maybe the cap being determined by some skills/stats, i.e lore and charisma?).
The instruments can then either be played via the key-to-note system that exists right now, or "play a song" that draws the information from the parchment, and automatically plays the notes according to the MML script.
The current instruments that exist could easily be adapted to MML, and it would provide room to add new ones in the future.
Also, it would be very cute. Please consider.