So I have a theory after seeing the past few Nintendo devices.
In the engineers room, they have 2 giant spin-wheels. One of them is divided into "console/portable" and the other one is "gimmick" which has items like "More Screens, More Dimensions, Spec Bump, Larger Size, Smaller Size, Invent-a-gimmick".
Every year, the engineers at Nintendo spin both of the wheels, and implement whatever the wheel falls on. This year it was Console and "more screens".
I'm waiting for the 4-Screened 3DS (Affectionately Called the 3DS2DD), or the Dual Screened Wii U (The Wii U2.. Sponsored by Bono?).. or the 3D, Dual Screen, Wii U.
Also, I heard Nintendo will release an improvement in their matchmaking. Instead of friend codes, you will be able to mail slips of paper with a QR code printed on them for users to take a picture with their 3DS and sync to their Wii U to add you as a friend.