A new rumor on a French Nintendo fansite has popped up stating that Nintendo is working on a new cartridge prototype that will allow streaming between the Wii U and the Nintendo 3DS. This cartridge will allow the Nintendo 3DS to function as a second Wii U gamepad when paired with the Circle Pad Pro, which is a 3DS accessory that gives an extra analog stick.
This news is especially promising considering just two years ago, Siliconera unearthed a new patent for Nintendo DS cartridge design that has not been previously utilized. You can see in the image below that the cartridge clearly sticks out of the Nintendo DS, so it might include some sort of receiver to pair the Nintendo 3DS with the Wii U if this rumor turns out to be true. Keep in mind that this is purely speculation at this point and no confirmation has been given from Nintendo as to the validity of this rumor.