Someone posted a picture of a cardboard insert into a "Marlin" case they bought on Amazon. Of course its just a filler image, because the cutout of the earpiece is off from latest leaks but there is something mysteriously on the front at the bottom.
First, people said it was a logo...the cardboard cutout suggests its an oval shaped button.
I'm hoping its another fingerprint scanner that supports gestures. Not only could you unlock your phone or verify for android pay, you could launch apps or access shortcuts using gestures. It's extremely helpful and allows the user to keep their fingers off the screen more.