This is one thing I've come back to over and over since the original GC Midlander thread. Before all the Ivalice references were dragged into this, Eorzea was based on Greece. If you consider the Twelve as the Olympians, they sit atop two fallen orders of gods.
First, the Primordials. In Eorzea, these would be represented as the forces from which all things were generated, and originally it seems that these are the beings that the Beastmen are calling from the aether; it's what they truly believe the primals to be. This is later proven to be wrong, but you have a manifestation of the idea nonetheless.
But what about the Titans? Beings cast down by the gods who came after - the new order. It works on a lot of levels (primarily because what we have to go on is so vague, of course), but even comparing the leader of the Titans to Zodiark seems like a slam dunk at a superficial glance. Cronus was the youngest of the first generation of Titans, Zodiark was the youngest of Ivalice's gods. Cronus led a Golden Age with no law, Zodiark was so strong that all things before him were undone, leaving him the King of Precepts. Cronus and his followers were cast into an abyss for their transgressions, Zodiark and the Ascians seem to be waging their war utilizing the void. Cronus is often equated or confused with Chronus, the god of time, Zodiark has power over time.
Rather than being the Twelve, the Ascians would be the gods they overthrew (whose first generation was also twelve). All of those similarities could be what led SE to invoke Zodiark from Ivalice for the Ascian god's name in ARR to begin with, considering he could then be connected to Ophiuchus, the thirteenth zodiac (Alpha Star: Sabik). And before Zodiark (XII), the constellation was was represented Elidibus (Tactics), whose form was a serpent. Ophiuchus bears the serpent constellation Serpens (Alpha Star: Unukalhai). Lots of connections. (Lots of coincidences...?)
I'm just stirring the pot, though. When things are so open-ended that you can barely take any basics for a granted, I hesitate to struggle with the specifics. It's a time vampire. (Damn you, Zodiark.)