In the future after you retire from adventuring, you and Tenzen become best buddies, you are training his teen/young adult daughter Iroha as your pupil, and the Emptiness comes back via plot shenanigans and basically takes over the world. You and Tenzen use Phoenix to send Iroha back in time to warn your younger (present day) self about whats happening and how to stop it. Unfortunately the Emptiness has also sent back a team of baddies to stop you. I'm skipping over a LOT of fluff here but suffice to say you go on a world tour and reunite with pretty much every major character through the history of the game before it comes to a crazy-even-for-Final-Fantasy final act where you become a god (two gods, actually), hop a few dimensions, kill a few bosses, and finally...FINALLY meet Altana face to face...when the game throws you a twist and you figure out that it WASN'T the Emptiness that destroyed the world after all but rather the interdimensional/multi-FF game appearing villain The Void and/or Cloud of Darkness who was pulling the strings. Vanadiel was kept safe because of its constant conflict but because you have kicked so much ass over the years killing bad guys, it swung too far into the light and it attracted the attention of the Void who wants to eat it up/destroy it. You get a scene with basically every avatar in the game in a LOTR style "And you have my axe!" moment where they give you their power (and yes, Phoenix finally appears as an avatar instead of a sword) and you go kill the last boss. Iroha disappears off to FF14, the song plays, long credits, game devs thank you in the credits for your years of playing, etc. And thats about it.