There was a rumor about an NX handheld, that had performance closest to XB1, but that it could stream gameplay or act as a source that streams footage to your TV through a wireless HDMI dongle.
Sounds pretty legit since Polaris would allow a 20 watt GPU to produce 1.3TFlops and considering AMD's Puma CPU was already able to hit 1.6Ghz, with the same core performance and core count as PS4 on 20 watts to it makes logical sense, if Zen is a 14nm part, with a redesigned architecture then it could potentially use less than half the power of Puma.
Costs would likely be really cheap too considering how many dies Nintendo could get from each wafer.
Notebook batteries have to contend with more energy demand than that would need once you take the full specs of a system into consideration. Even the possibility of putting something more energy hungry than Wii U in a handheld isn't unbelievable.
Here's the link to the rumor:
http://wccftech.com/rumor-nintendo-nx-power-closest-xbox-run-ps4xo-games-modification/
The whole closest to XB1 thing could either mean it's just over half the power of XB1, half way between PS4 and XB1 or somewhere inbetween. Technically a handheld with even PS4 levels of power could exist, it just needs a decent enough battery to power it, but laptops have those, so there's no reason why Nintendo couldn't one in a mobile console/handheld device, besides form factor issues.
I don't know if this whole wccftech rumor has been proven false or whether it's among the unknowns, but it's technically possible and worth a mention.
