Repeat all the previous steps for the second shoe. 

• Eliminate caps for axle, so only TPU is needed, and to make assembly easier.
• Use hex nuts instead of square nuts.
• See if I can find a cheaper and more available replacement for the rectangular bars.
• Try improving the design so the shoe can bend more. This would make walking more natural and eliminate the need for the foot flap.
• Make a design that doesn't require a regular shoe. It would be smaller, so less material and less weight.
• Try using neoprene tubes with the wheels to reduce noise further.
• Electronics for feet tracking (wheel encoders, IMUs).
• Motorized version.
• Maybe make width adjustable.
• Maybe make length easier to adjust.