Idea: Trading Post
Is placed opposite the items in the shop, and you can trade in the notes that change the instruments' sounds for an ability with the same level as however many notes you put in (1 note: level 1, 2 notes: level 2, etc.) but to stop this becoming too OP, for level 3+ abilities, you also need to give coins as well as notes. This can only be used once per shop room.
Oh yeah, and some way to customise the colour/shape of the dots and the player icon would be pretty neat.