At one point there were rumours of an apple branded fitness band like a Fitbit. Sadly the rumours have yet to come true.
I don't think they will honestly. The Apple Watch is a phenomenal product because of how frighteningly indispensable it is to those who use it. It's far more deeply rooted in its users' lives than the iPhone or Mac are, which are still replaceable to a large degree if you really get sick of them one day.
The AW to me is the ultimate lock-in to the Apple Ecosystem so I don't see Tim & Co. shaking up the formula one bit. Especially releasing a product that performs all the most-useful fitness oriented features of the AW. Would really love to be proven wrong about that though some day!