Touch ID
One of the most significant rumors about the Apple Watch S6 has to be the Touch ID integrated on the Digital Crown. Having an extra layer of protection would be nice and useful for many people, and quicker than typing in a passcode.
As of right now, a simple move of your wrist can show eavesdroppers private information on your Apple Watch. With a lock screen that only your finger can unlock, that wouldn’t be a problem anymore.