My personal minimum if I'm Nintendo, is I would ask AMD for a portable chip that can run a port of any Wii U @ 720p.
That means in portable form it should be able to run Splatoon, the new Zelda, Mario Kart 8, whatever. I think it's possible.
The Wii U chip consumes 33 watts running a game off a disc drive (so probably actually 30 watts or so) @ 40nm, tinker with that, throw out the IBM CPU cores for something more modern, start with a 14nm chip, obviously no disc drive ... you probably start to get down into the 7-8 watts per hour usage range without much fuss.
Keep in mind the Wii U chip is also old as fark, it's 2010 tech, AMD likely has learned a lot about power efficiency since then too.
If you can get better than Wii U power ... fantastic.
This way Nintendo's developers can basically continue developing games of the same scope without any fuss. Then for the super-duper NX console, just take that same game and scale it up to 1080P (or beyond even .... 2K? 4K? depending on the complexity of the game), add some graphical effects and voila. Now you have games that function on both handheld and console without Nintendo's designers being compromised too much in what they do. I think Nintendo's designers probably would be fine working on Wii U level visuals for the next 5-10 years.
This would allow Nintendo to port games like Xenoblade X, Donkey Kong Country: TF, Bayonetta 2, etc. to the next-gen portable, which would allow these games that were criminally underplayed by most people to find a wider audience that they deserved.
