There is still a lot of speculation over what the Nintendo NX will be, largely because the amount of solid information we have on it is close to nil. The idea of it being a home/handheld hybrid has been floated around for a while, but thus far, nothing has turned up.
It does seem once it is announced though, it will be something a little different. Speaking to Asashi Shimbun (as translated by Perfectly Nintendo), CEO Tatsumi Kimishima said that the NX is not a strict successor to the Wii U. or the 3DS. When asked about the affect on Wii U sales when the NX comes out, he said:
The NX is neither the successor to the Wii U nor to the 3DS. It's a new way of playing games, which I think will have a larger impact than the Wii U, but I don't feel it's a pure replacement for the Wii U.
That is… interesting. This could just be PR speak to try and keep people buying Wii Us until the announcement, but if not, it would seem to suggest that this is something a little outside the handheld and console model. Now it is just a case of when the thing will finally make an appearance.