They start development right after the current generation. Always.
I think it will be introduced E3 2016 and then released Holiday 2017.
You realize the main reason Wii U didn't do well was because of lack of good games? That is changing, and that means more Wii U will sell now.
No, the reason why the Wii U isn't doing well is because of multiple reasons:
1) The lack of games like you mentioned: This is due to no one having a Wii U, which makes third party developers not want to make any games for it, and the ones that do make half baked ports for it due to the lack of power.
2) The price the Wii U could have been a better deal for this price if the gamepad never existed and used the pro controller and the money that went into the gamepad to make it have more power.
3) The name and lack of marketing even today some people still call the Wii U just "Wii" because it has "Wii" in it, the name is confusing and stupid. Nintendo doesn't know how to market it because the gamepad is useless in almost every way, so all they do is try to loosey work with what they have... "Oh look, we have smash bros and mario kart, go buy it now!"...