A countdown teaser website for an unknown Square Enix game for Android and iOS is currently up right now.
The page in question features weapons and related quotes from an assortment of Square Enix games such as Legend of Mana, Kai-Ri-Sei Million Arthur, NieR: Automata, Romancing SaGa and Seiken Densetsu 2. Given that they are specifically weapons, one would imagine that Square Enix is trying its hand at making a game that is similar to Touken Ranbu. Given that the latter has got the female gamer market cornered, Square Enix may instead focus on the other end of the spectrum.
In any case, do expect more information when the countdown hits 20 September 2017 for the reveal. With the Tokyo Game Show taking place the day after that, we are going to guess that is when Square Enix will divulge even more on the game.