One problem that I have with treadmills are that they are simply boring. You literally stand on a moving belt, and run in place. I think a good redesign of a treadmill would be the additional of a screen, with a simulation of running somewhere, to someplace.
This is already done with the bicycle machines. There are many ‘simulators’ where people ride bikes to exercise, but at the same time steer the stationary bike and view their actions through a connected screen. A similar approach could be used to redesign the treadmill. There could be a screen on the treadmill, with the simulation of a character running through different sceneries, to provide interaction.
This way, it won’t just be exercise. It will be exercise linked to a game, which would make people want to exercise more.