I'm not sure this would be nearly as exciting as a genuine OSRS gold game, like Guitar Hero or Rock Band because of the game's inherent tick system. This sets the game off with a bad start because of the tick system that people are used to. "Demystifying" how ticks work system shouldn't be considered as a good thing as well. It's an extremely significant engine issue that's difficult to overcome. Bard isn't required in the least bit to educate people about what a game tick actually is. It takes a few seconds and can be demonstrated by having someone walk between 2 tiles or using a metronome at 100BPM. Don't overlook the importance of tempo in composition either. In music common to the genre of fantasy, time is an essential tool to create mood and ambience. It's a catch-22 problem. An AFK skill will provide you with better music , but it's extremely boring to learn or a rhythm game would make users to share the same experience of music that would be difficult to listen to for long.
The goal of everyone is to increase efficiency when they train to improve their performance, which is a huge issue. So, they'll be doing the same thing over and over again even if there's only a 1% difference. I'm not able to listen to my favourite music too often or get bored of it, regardless of how well-crafted it may be. The ability should be so rapid to develop to tackle this issue that it shouldn't be a skill in the first in the first place. Otherwise, it's either a gruelling bore, or with audio off it defeats the purpose of the music-based skill.
It's also true that music is already assigned to quests. This is a deliberate choice, because in the world of fantasy it is commonplace for in-world music to be utilized as a storytelling tool (LotR is my personal favorite) as well as, of course, music can emphasise important moments in the story if required. The music will lose all significance when it becomes something you need to "get to," repeated ad infinitum as a roadblock to the next XP drop or milestone in level. It's disingenuous of me to label it an opportunity missed. It's just wrong and could do a tremendous detriment for the music composition.
The fact that everyone would be required to learn this skill is a problem. It's like the elective classes students must attend that are supposed to be enjoyable however they result in being boring due to the fact that they transform something that might be fun into assignments to get through or piles of work. Instead of school work and homework, the aim of the best skills is to make it as focused on the experience it is possible despite the immense pressure from players to achieve an endpoint. This is a huge challenge that lot of game designers must overcome because players naturally seek to get the most fun out of games; therefore, a great skill should not be confined to a single element necessary for it to be enjoyed and fully immersed in it , but is so boring that it's disengaged (music).
It's good because it's inherently good, which means mechanics that work without a hitch regardless of the choices of players. It should make sense both systemically and thematically, not just "be possible to train with audio settings off" or "be put into the limiting game engine, in whatever state it is, just because." That's the way that music-based skills will always feel, considering the limitations that hinder them. If they can that, then the music should be relevant to the skill. That is impossible to do properly in OSRS.
Further problems that aren't all that important given the main issues: players have different musical skills, so there's a need for a difficulty system that covers everything with a balanced system for some or even less XP. The keyboard and mouse are not good instrument for virtual music in a rhythm game, and there are numerous instruments with different performance metrics in case accuracy is important. There are many old-fashioned instruments in the pictures such as the sackbut, that dates back to the 15th century in France (shoot for the dark times). Rewards can be anything that is RS gold however I'm not a fan of how sensitive they are to the economy or the meta-skilling method; it's like they're self-aware of being problematic before they even exist.
