There is a chance that Final Fantasy XIV Online may be headed to Xbox and Nintendo platforms. In an interview with game director Naoki Yoshida by Gamespot, it was stated that discussions with Microsoft and Nintendo, as well as Google, are currently underway.
It was stressed in a previous interview that Final Fantasy XIV needs to support cross-play to have its player-bases united and offer the best experience. Before it was only possible with PS4 and PC. With Sony and Nintendo beginning to allow cross-play with competing platforms for games like Fortnite and Rocket League, there are higher chances for the game to move to these platforms.
Yoshida even stated in his interview, "So as long as the regulations with the counterpart hardware companies … we can actually pass that, then the possibility, the chance is quite high" [of Final Fantasy XIV coming to Xbox and Nintendo systems].
However, this is mostly speculation. According to Yoshida, negotiations with the other platforms are still going on. He reassured though that once there are details, there will be an announcement.
