I read an interesting theory elsewhere that perhaps the NX isn't necessarily a hardware console, but rather some kind of development platform. I mean, Nintendo has said nothing about what the NX is supposed to be, and have only vaguely implied that it might be hardware (very, very vaguely). It could make more sense to consider that it may be a unified development platform meant to simplify development between their next devices (both handheld and home console) in order to decrease development time, as well as encourage the development of original content from other developers on Nintendo consoles. It will likely be employed at the end of the Wii U/3DS life, but meant primarily for their next gen hardware.
I imagine it will probably be announced alongside their next gen hardware lineup, or at least the handheld, assuming this theory is correct. The 3DS and the Wii U are both clearly on the way out, and I can't see Nintendo putting off a new hardware release for either past 2017.