There is a mystery game in Nintendo's half-year financial results presentation, which was released in full today. It's called Miitopia, it's billed as a "major title", and it'll be out in Japan before the end of the year - but Nintendo has announced nothing more than a name, which appeared at the end of a September Nintendo Direct presentation. Mysterious indeed.
(If you're curious, the other "major titles" for the holiday season in Japan are Monster Hunter Stories, Yo-Kai Watch 3 Sukiyaki [both third-party], and Momotaru Dentetsu 2017 Tachiagare Nippon!!, which is like Japanese monopoly crossed with a travel sim.)