I put together my first prototype today.
By reading a potentiometer signal, via Arduino and a Processing sketch, I was able to build a simple dial to change the opacity of an ellipse.
It feels good manipulating the alpha value this way, much more direct than dragging a slider or even using arrow keys.
A useful first step. But one lone function doesn't give a great deal of insight into how working extensively with such controls would feel. I need to build out more controls and versatility to begin understanding that.