It would be extremely unlikely that Nintendo would release a smartphone, but if the company did then it could look like the one that artist Pierre Cerveau has created. It is titled the Smart Boy and certainly looks rather fancy, but minimalistic at the same time. Clearly it has been inspired by the iconic Game Boy. The Smart Boy would also come with a Game Bat controller that would slide into the phone to enable you to use traditional controls, which would certainly be handy. You can check out all the mock-up images of the Smart Boy, here.
Thanks, Dustin W