I'm taking a class in creative computing using Processing and Arduino and one thing that I've been hankering for the entire semester is a GUI library. When I see computer graphics created using Cinder, my face goes green with envy. I would love to be able to add beautifully-designed sliders, buttons, switches (and MORE) to my Processing sketches. I believe that this would also encourage Processing users to make their sketches interactive, and make data visualization a lot more fun and intuitive.
I have used the TKinter library in Python so I guess I would start with a basic toolkit similar to that.