1. Make the surface of the ball a multitouch trackpad surface so you could pinch zoom in and out and two finger scroll up and down, all on the ball itself. Make it only work when the ball is not pressed on hard enough to move.
2. Make the ball slightly squeezable, which registers as a click. That way you could roll it around on your desk or on the base. This honestly would also be a great kids computer toy so they don't have to get their hands near the device itself.
3. Add electromagnets inside on all sides and place the ball on a magnetic trackpad or base. When squeezed it would turn on the downward-facing electromagnet in the ball to make it harder to move, so you could engage in finer detail work.
Probably a materials challenge to do any of these but it would be really cool to see these new input frontiers explored.