Ok, I know this is is REALLY going out on a limb and somewhat wishful thinking.

Picture this...
A cross over event with the headline of "3 mysterious warriors appear." Your character has to first figure out who and where these guys came from. Through a series of quests, you find these warriors are actually time travelers named Chrono, Lucca and Marle and they've arrived in this world to stop a world-ending threat that goes by the name of Lavos. You all end up having to work together to defeat Lavos with Lavos being some kind of large-scale open world FATE on the level of an Odin or Behemoth fight.

I believe it fits with Yoshi's guideline of "wanting a game to come to it's conclusion before trying to make it fit within FFXIV's world." It'd be similar to the Lightning event with her hoping worlds and whatnot...except with characters we actually care about. (Sorry Lighting fans -_-)