Rumor has been, as the summary says, that the NX will be a console/handheld hybrid. However, that's not a particularly novel idea, as it's basically what the Nvidia SHIELD is, and I thought up the idea years ago thinking it obvious. Nintendo said they didn't want to reveal what their system was too early, since they didn't want to be copied by competitors.
I don't blame them much, given that Sony and Microsoft jumped on the motion-control bandwagon after the success of the Wii... but being coy doesn't give them any additional lead time. If Sony/Microsoft believe the rumors, and want to do what Nintendo is doing because it'll surely lead to success (not a guarantee given the Wii U), then they could start designing a hybrid device before the official reveal. If they're going to take a wait-and-see approach to see if this Nintendo idea pans out and has any success, then it doesn't matter how early the concept is revealed. Perhaps Nintendo is aping Apple by being secretive and then having a big reveal shortly (5 months) before it's available for purchase, so that all of their marketing momentum is concentrated within a short span of time. Considering Pokemon is traditionally a handheld series, and if this is a handheld/console hybrid, there could plausibly be a mainline Pokemon game on it. There might be a 3ds-like resolution when portable, and you plug it in to a TV for higher resolution. The console has been in the works so long I don't think it'll have much/any support for 4k.
If they say "Surprise! we were working on self-contained portable VR this whole time! and you can plug it in so the chips increase their clock speed for better graphics. At $299!" then I'll be impressed.