I've always wanted to try a theremin and potentially design an instrument that works like that with gesture control!
People seem to say that certain instruments work therapeutically, although I have no idea whether that's true or not. I picked up a Tibetan singing bowl a few years ago in a creepy little town near me called Cassadaga (if you know, you know) just because I thought it looked really beautiful, but I guess 'legend' has it that it's super good for inducing trance-like states and meditation, which could be seen as therapeutic as well.
I'm not so sure about all of that, but it can't hurt to pretend! Still, trying to make a sample pack out of a one-note instrument is the greatest challenge I've ever faced in my life. It's right up there with my didgeridoo.