According to a rumor that's flying around the French website Nintendo Master, Nintendo is working on a cartridge that will allow for streaming between the Wii U and the Nintendo 3DS. What's that mean, exactly? Pretty much nothing until Nintendo confirms otherwise, but theoretically it might mean that the company is looking for a way to let the 3DS function as an additional Wii U controller.
Though the 3DS would have to be equipped with a Slider Pad to work as a Wii U controller, the idea is still a good one. Currently, there's no way to buy the Wii U tablet controller a la carte, and though that will change in time, it's not likely to be a cheap purchase. Given that the Wii U already has a great deal of Wii compatibility woven into its hardware, it'd be nice to see 3DS get wrapped up in that embrace, too.
Incidentally, back in 2010, SiliconEra noted that Nintendo filed a patent for a large Nintendo DS cartridge. Has Nintendo revised/revived the concept for the 3DS and Wii U? Again, until Nintendo says "Sure," this one gets filed under "Rumor," since companies file patents all the time and not all of them amount to anything--but we'll make a copy so we can also file it under "Cool Things That Should Happen." Wii U Daily, Nintendo Master, SiliconEra