Let me start off by saying everything written on this particular post is just a bunch of speculation and rumors. With that said, if the past is any indication of the future, said rumors tend to be accurate.
The next iPhone (iPhone 8?) is supposed be getting rid of the front home button. This means that the Touch ID sensor is going away as well, or it is being repositioned elsewhere. If this iPhone 8 video is accurate, then the Touch ID sensor is going to be on the back of the phone.
This 10-second iPhone 8 Touch ID video came from Weibo (China’s version of Twitter). It shows someone pressing their finger to the sensor which triggers a manufacturing diagnostics machine.
Some reasons to believe this video are the fact that there is the Apple logo, the vertical dual camera system, and similarity to previous leaks. Time will tell.