I think NX might stand for "new experience", in which case whatever it is, it will probably have a new control style or in another way be something entirely new and unique. It will be interesting to see.
They've said that it's not meant to replace the existing consoles but to complement them, so it's hard to imagine what it might be.
That the NX is not supposed to replace the Wii U should be most concerning if they don't have a replacement in development.