Technically there already is musical instruments in the game that players can access (not including the BRD's bowharp of course ) - they're furnishings for FC/player housing and are based on similar instruments seen as background items in Haukke Manor. You can click on them and they play a few eerie chords corresponding to what instrument it is (also, surprisingly, the manor harpsichord.... sounds like a piano, which any musician should be able to tell you is a totally different instrument with a completely different sound!).

A modification which would allow players to actually play them with actual musical notation sounds like an amazing idea, especially as several earlier FF games allowed you to do this (FFV, VII and VIII spring to mind).