Perhaps this has been asked before, but it's really something I don't want to pass up... Please, do not let the tiny negatives cloud the bigger picture in this, I seriously think us being able to play instruments and make our own notes with it will for one, EASILY add so much replayability to the game, but not just that, also life.
I was no fan of Mabinogi personally, that's just my opinion but to be fair, I did not play it when it came out, I played it several years after. But one thing that really appealed to me was this huge customization and player craftable art in this game, was it abused? Sometimes it was, but not often, not at all, the players I encountered when playing music were more focused on playing something actually good rather than disturb people. I know there's more games out there that have this feature, but I cannot remember off the top of my head.
I recall there are other small but very big things you guys added, like fishing if I'm not horribly mistaken. If those can be added here, adding in the ability to play music with customizable notes would be just fine. Small minigames, like an elder scrolls spin on chess or go would also be something I would be very happy to see, just the little things that would add so much life to any game.
Really, the only argument I can think of against this is that it'd be abused, but in hindsight, there are other features in all mmos that get abused, like the chat system, and yet I see no limits imposed against it, if you don't like the asshat, you are given the choice to block her/him, that can be done the same way to those who play instruments with the intent of annoying people as well.