New consoles go into R&D almost immediately after the latest one is released. From what I understand, the NX isn't even necessarily a console, it could be a handheld, which makes more sense due to the 3DS being older. Considering they aren't even going to share details about it until next year it wouldn't be until 2017 at the earliest until it's released, which makes sense with the Wii U and/or 3DS lifespan. The PS5 and the Xbox Two are both also being worked on, for what it's worth, yet the current consoles are stated to have a 10 year lifespan. That doesn't mean they're on their way to stores.
It was only inevitaburuuuu that Nintendo would start developing mobile games, contrary to what they've said in the past. There's too much money to be made there. Personally I don't care at all because I don't even have a smart phone and if I did, I wouldn't be playing games on it.
I love the Wii U but damn Nintendo really dropped the ball with it, hoping that just using the Wii brand was enough to propel popularity and negating gamepad features. But mostly pretty much distancing themselves from third parties entirely is what killed it. Lots of third parties really screwed things over though, with incredibly late ports, often with features arbitrarily missing, and then going "Look! Our games sell less on the Wii U!" blaming it on Wii U owners, and not the fact that they released a gimped version of a game everyone bought a year before. So hopefully Nintendo decides to stop relying on gimmicks and works on repairing relations.