You know what would be fun? If you added multiplayer mode.
Co-op mode: Harmonize with your ally! You get twice as many instruments but all other drops are just as common. Player 2 would have to teleport to player 1 when he enters a new room though...
Versus mode: each player gets 8 notes, 2 options for square upgrade, 2 options for hexagon upgrade, 2 options for diamond upgrade, and 5 options for stat upgrades (you get 3 of them though) and then battles it out!