The Iphone 7 already contains the major updates to the guts of the phone. The next one is refinements of the guts and major update of the exterior.

It is fairly obvious they are going OLED.

All glass body.

I'm going to go out on a limb and guess the finger sensor gets embedded on the back of the body.

I think there will be a super high priced version with ceramic body, with the glass as the base option.

Change in screen size as well. the 4.5 probably becomes 5", enabled by the move of the sensor to the back home button.

And it will of course be the "best ever".