Years ago, Tetsuya Nomura showed a trailer regarding a Final Fantasy Versus XIIIgameplay. which allowed players to switch characters in a battle. In an event held five days ago for the one year anniversary for Final Fantasy XV launch, Hajime Tabata announced a few upcoming updates for Final Fantasy XV and the future plan for 2018. This included the above mentioned interchangeable character in battle system for Final Fantasy XV. It will be available with an update called Final Fantasy Online Comrade which is set to be release before Episode Ignis. The might be an future Episode Ardyn Izunia DLC coming which will continue the story of Lunfreya as well. Hajime Tabata and the Team for Final Fantasy XV will continue updating the game with more content like the Assassin Creed Festival and Moogle Chocobo Carnival.
Here is the livestream from Square Enix
livestream in japanese language