Xiaomi is reportedly working on a secondary display device with a quad-camera setup. And this will most probably their Mi Mix series. The company has patented the design where the phone sports a small secondary display at the back next to the camera module.
Xiaomi Secondary Display Device
The above patent clearly shows us a completely bezel-less design. As per the patent, there is a secondary display at the rear portion of the phone along with the camera module. The square-shaped camera module has a space for 4 sensors which means there could be a quad-camera setup.
As there is no camera setup at the front side of the phone. We can predict that the phone will take benefit of quad-camera and the secondary display for selfies as well.
As well the secondary display is as small as the quad-camera housing. So most probably it will be used to show caller ID and notification alerts when the phone is placed facing down.