It doesn't need to be balanced in fact it needs to be exactly the opposite, it needs to have an extremely heavy floating weight somehow attached to X and Y axis motors. When the X motor was actuated the torque would try to lift the weight causing the ball to fall in the direction it was moved, and you'd need some pretty good control loops to let the thing go anywhere in a controlled fashion, breaking would be difficult as well because if you try to stop too fast the weight will flip over and the entire system will lose orientation. You would need to know the X and Y position of the floating weight and have an independant X/Y/Z accelerometer floating inside the electronics package for feedback. I've had the same idea in my head for several years, and I'm sure I'm not the first person to think of it but I've not the resources right now to even try basic experiments right now. The larger the ball the easier the prototyping and stability would be, you could start with a gerbil ball for prototyping, it's all sealed up and has an access port just waiting for you to put moving bits, best thing is you can SEE what's going on inside.