A trademark registered by Nintendo was recently found over in Japan. The trademark is for a Wii Fit Plus which appears it could be a follow on to the Wii Fit game. Not many details are to be found in the trademark although there is a designed logo which would indicate that Nintendo could well be working on a new product rather then just protecting a potential future product.
The Wii Fit Plus could be a number of things such as a new game for the Wii Fit or is could be a complete new balance board with more accurate hardware or even more possible, extra features to allow for more accurate and a wider variety of exercises.
For now the Wii Fit Plus is just an unknown, but as usual, expect more information to follow in the coming months.