Nintendo still isn't confirming anything, but multiple sources point to the ultimate evolution of the Wii U.
The NX codename may refer to a dockable portable console, according to multiple sources via Eurogamer.
The main processing power of the unit will be contained in a standalone portable system. The unit will dock into a connector hooked up to the TV for home use, and local multiplayer is handled by detachable control units on the side. Games will run either digitally or in a cartridge that can store up to 32GB.
This form factor would run on Nvidia's Tegra X1 chip, a chip currently used in high end tablets such as Google's Pixel C convertible tablet. Due to such a major architecture change, there would be no backwards compatibility with Wii U or 3DS.
Nintendo has not commented on any of the above information.