Hi,
a Friend of mine is working for Nintendo. He has access to Nintendo NX Development Kits. I'm myself a big Nintendo Fan and because he knows that, he told me something about Nintendo NX. And what he told me was that you are able to control Nintendo NX games with your Smartphone or Tablet that you already own. He also said that there will be a standard controller too, but you can as said before also use your own Smartphones and Tablets to control Nintendo NX games. Nintendo also plans specific apps and games for Smartphones and Tablets that will be able to interact with Nintendo NX.
To me it sounds like it is meant to be like the Nintendo Wii U, which also has a GamePad with a screen on it.
Of course I can't say my source, because he would get fired if I would ;) You can believe it ot not, but I trust my Friends! And that's why I consider this not as a RUMOR, I consider it as a true FACT ;) He also said it's okay when I share it with the world, as long as I don't mention his name or where at Nintendo he is working!