Well, we don't know anything about Myths of the Realm other than it'll involve the Twelve somehow. We may not even end up fighting THEM but whatever ancient (lowercase "a" ancient) threat they fought to be known as one of the Twelve. Or we might fight a manifestation of the Twelve brought about because prayer + aether = Primal (after all, Louisoix's spell was supposed to SUMMON THE TWELVE, the plan was just to stop it before that happened and use the aether to trap Bahamut). Or since it's a 24-man, the fights are of a different scale. I don't think the Twelve NEED to be "fearsome", anyway; I mean, in the background they're still benevolent deities that people pray to for succor.

I think a main point of FFXIV is that there ARE no gods, not as we understand them, but belief is still a thing and there's power behind belief (American Gods made a similar point about the symbiotic nature of belief, with Gods feeding off the peoples' belief and answering their prayers in kind). Personally I'm much more intrigued about the story of the raid and clearing up the mysteries behind the origins of the Twelve.