I'm guessing it's a hybrid (dock able) home and portable that will support software specific to the system (maybe slightly more powerful than wiiu) and have a tablet form factor. I also imagine it has the added bonus of playing wiiu games (both at home and on the go- maybe digital versions of these games so no disc drive is needed) and ds games (also both at home and on the go). I also think it might act as an added controller for wiiu (but better with multi touch screen, added functionality etc) so you can use it as a second controller/screen if you already have that system.
But those are just safe bets. As far as functionality it still has to be unique to have marketability beyond the wiiu. Who knows what they end up making but personally I still think a very well done local multiplayer focused amiibo/toy based augmented reality experience would be perfect for them (tablet screen not head mounted). I can imagine a bunch of kids sitting around each other looking in on a virtual augmented reality smash bros battle/pokemon battle, etc, where each system acts as a window into the world and the toys/amiibos are the primary method of content delivery.