AURA Strap 2 for Apple Watch
AURA Strap 2 brings body composition and water level tracking right to your Apple Watch. Measure how much your fat, muscles, water level, and many more parameters change over the workout or diet process. Build the body of your dreams with the new Strap... More here.
There are so many variations on this idea, but none of them has stuck to date. Personally I would like to see Apple make a much smaller tracker only device, but I cannot see Apple doing that at any point.