I really feel like we're being led towards an FFIII or V "two worlds split apart" revelation. The whole thing with the Rejoining, whatever Urianger was reading, and the WoD having fought numerous Primals but us not being aware of it seems to point to the idea that the WoD is us, but from the "other side", where Zodiark takes the place of Hydaelyn as the massive crystal in the heart of the world.
The only thing that concerns me is the WoD flashback where he very clearly uses a weapon of Light. It could be that he used to be against the Ascians but was exposed to some truth or other that swayed him to their side.
Maybe they'll be as lame as to go down the Alternate Universe rabbit hole like Abyssea in XI, and WoD took all the same steps as us in his world up until some crucial decision after beating Lahabrea (assuming thats what that flashback was) when he was swayed to the Ascians' side. I mean, As Goes Light, So Goes Darkness seems to indicate some degree of parity in what we've been through.
I'm thinking this is all heading towards a "The Ascians are trying to do the Right Thing in a Bad Way or for Bad Reasons" revelation. Like, the Rejoining might not be a terrible thing, but trying to achieve it with seven events of mass death and chaos is the wrong way to go about that. Likewise, if they're trying to weaken Hydaelyn so Zodiark can predominate her (because they're tempered by Zodiark?), maybe there's a way to join the two in a more equal measure. Elidibus certainly seems concerned with balance more so than the others who are all "LORD ZODIARK SHALL RULE ALL" all the time.
I'm also assuming the moon is somehow the gateway between worlds for no reason other than that's where we first saw the WoD.