5" or more OLED screen with no breaks in it that you can fold in half would be great.
Avoids the unwieldiness of recent phones/phablets.
However, I have yet to see an example of one flexing enough to enable a thin folded form. Just being able to bend it seems a bit pointless.
Impreza WRX STi Spec-C V-Limited Toshi Arai Edition/ GTR/ Panda 100HP Pandamonium/ 316d