I think this idea might be dumb, but I feel like there could be a potential to create a homebrew app for the 3DS in which it emulates the Wii Fit U Meter accesory.
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I believe almost everybody has a smartphone that natively tracks their footstep. They could make an app that gathers steps of the day from the health app and sends it to the Wii U
The point is not for actual use but just for fun...