Alright, so the other day I was listening to some awesome Zelda music covers, and I really awesome idea popped into my head. It sort of coincides with some things I'm really interested. Namely, I love hearing Zelda music played by skilled violinists. So it's no surprise that my earlier answer...
Some sort of acoustic string instrument, or a wind instrument. But I think a string instrument would be better for what I'm imagining. I'd like to see some functionality out of that touch pad. You could strum strings with one hand, and hold the controller with the other.