No matter what it is, 2016 is too soon imo.
2017 or 18 would be my guess.

Black Friday 2017 for US and Dec 2017 for Japan and Europe.

2016. Incase people have been living under a rock, the Wii U has been effectively let go for a year. It's already dead. They're not "getting ready to kill off the Wii U." They've already killed it, and it's just hemorrhaging it's last breaths by rolling out the last of the games that were announced when it launched.

XCX, SMTxFE, and Zelda U. It's last three big games are games that were all announced at the same time, two months after the system launched. It's over. The Wii U's biggest new holiday title was scrambled together in a year. It's done. All they are doing is doling out the rest of the games that have been in development since the Wii U's future seemed brighter.

Worked fine for the market leader this generation. Working fine for the XBO, too. Not even close to too soon.

2017 first half

Hmm, you do have a point. Though it did have 8 to 9 months to hype it, which is quite a bit of time. If the NX is showed at E3, then maybe early 2017 would be good? If they at least market it well enough...

Also, I meant year and not here, how embarrassing... u_u;

Q4 2016 at the earliest.

The XBO was announced like weeks before E3, though, so....... Holiday 2016 is still fine. Especially since speculation has already started. The NX has already had it's hype started by Nintendo. If it comes out in Holidat 2016, that'll be like 18 months to hype it up. Mentioning it by name, at E3 in their direct watched by millions of people this year is hyping it up now. Neither Sony nor Microsoft did anything like that with the PS4 of XBO the year before they launched.

November 2016

Or DS & 3DS which were both released less than a year after being revealed.