im pretty sure its gonna be a dlc world. Almost every other game in the series with a secret ending was really obscure and foreshadowing the concept of the next big game in the series in someway. This is too obvious, and the fact that Yozora was a game in the game makes it seem like it'll be a DLC world - as if to make commentary that even the worlds in video games can be worlds in other realities.
Add to the fact that there were no FF characters in this game... I suspect that the world in Yozora will be a portal into the world of final fantasy so to speak.