Well... for one thing, the Shantoto (XI cross over) event kinda tell in a way that Eorzea & Vana'diel might be connected somehow, and XIV current time might be distant future of XI timeline. Would explain why there's many mobs design from XI were intact in XI. Heck... even Gigas, Sahagins from XI's Zilart expansion and Mamool Ja from Aht Urhgan expansion also exist in Eorzea. And let's not forget about the goblins & qiqirns who were usually found in cities doing their usual trading stuff.

We do even get treated with taste of XI's latest expansion which the existance of White Lion as PLD mount achievement.

Which makes me wonder, atm XI already has 2 crossover pair (that's XIV & DQX) events while XIV still has one. Wonder if SE will make another of this happening again in the future...