If only I had the money...
I would use magnetism to make this OD treadmill.
Make magnetic pads you can strap to your feet or shoes, and then make a deck with a grid of electro-magnets to stand and walk on.
You could then program the deck to keep the user in place when walking, by using the magnets to steer the person any direction.
It would work, but I have no money, electrotechnical - or programming skills.