I'd love more music in the game.
I know it's hard to come up with a theme tune that's not too repetitive (and I think that's been mentioned before) but if there were slightly different tunes for each location or each district, then it would really add atmosphere. It wouldn't even need to be music, even, just some background noise that gave you a sense of where you were (trees in the wind, echoey caves etc)
In addition, I'd love to not only be able to listen to music and play the odd block here and there, but learn to make and play instruments as well. Imagine fashioning a drum out of bubbles, or hollowing out a cloudberry and petrifying it to make an ethereal whistle.
We could make little bands and busk in busy places to raise currantcy.