After the success of the Wii wheel, (Mario Kart) balance board, (Wii Fit) and motion plus (Wii Sports Resort) I think it is safe to assume that Nintendo will release something new to help sell the Wii U. They have seen the success that add-ons can bring. Kinect helped the XB 360 a ton too. Nintendo even announced the "Vitality Sensor" which was canceled due to it not working as well as planned. (wonder what games they had planned?)
Anyway, my guess is as good as anyone else's but I'm going to go with haptic feedback support. Why? Back in 2011, if anyone remembers, rumors began stating that the Wii U would be called the "Feel" and would provide haptic feedback. This was obviously not true. The leaders of this type of tech "Vivitouch" and its "HD Feel" seem to be interested in bringing the tech to gamers. They've been to E3 for a few years now, showing off the "HD Feel" as well as a new headphone with "4D Sound". In 2012, it was rumored (again) that Nintendo would team up with vivitouch to include haptic feedback into the Gamepad, this was again untrue.
Fast forward to current time and the Wii U Gamepad (while cool IMO) still has yet to showcase something that gets the casuals excited. After some digging, I found this tweet from Vivitouch: