Though there is currently insufficient evidence to support any conjecture on Ascian origins past "wild speculation", an idea occurred to me that is supported by recent revelations in 2.3, doesn't conflict with any known evidence, and may simultaneously resolve other big mysteries. We have to make some leaps of logic to get to my conclusion (and the path we take to get there may be questionable at times), but the destination seems sound based on existing evidence.
We know that the Ascians pre-date Hydaelyn (Lahabrea's "parasite" monologue in The Praetorium). We now also know that Ramuh, and thus presumably the other primals, predate some point where a duality of Light and Dark was created (Ramuh's pre-fight speech). It logically follows, since Hydaelyn is an entity of Light, that Ramuh predates Hydaelyn as well. We can also make a reasonable leap of logic that the appearance of Hydaelyn coincides with the creation of this duality. It would then follow logically that the Ascians predate a duality of Light and Dark.
This is a rather significant conclusion. That Ascians predate darkness seems contrary to their nature as entities of darkness. This paradox is resolved if we assume the Ascians did not always exist as entities of darkness. This assumption would also provide us with a personal motivation for their crusade. Since it seems safe to assume that the arrival of Hydaelyn corresponds to the loss of Zodiark, the loss of Zodiark could also be attributed to the creation of this duality. Thus the restoration of Zodiark would also be the restoration of themselves and "the planet".
So what were Ascians before the duality of Light and Dark came to be? We don't really have much to go on, and anything we come up with is going to be highly speculative. But it wouldn't be any fun to stop now!
If we want to assume this question has an answer, there's only one answer that fits well. Given how the majority of the Ascian overlords have been named so far (scions of light from FF XII) we can deduce that Ascian leadership numbers about 12 (with Altima filling in the hole left by Ultima, plus the inclusion of Elidibus). This immediately leads us to The Twelve.
But Torquil! There could be 13 Ascians! Yeah okay...so Nald'Thal is 2 then (good old Nald Thal).
Going along with this "duality" theme I postulate that around the time of the 1st Umbral Era The Twelve were split into Light and Dark aspects with the Ascians being the dark aspects. This would make the Ascians similar to the Skeksis from the 80s film The Dark Crystal. Alternately, The Twelve are the light aspects and both they and the Ascians came from some unknown origin (Zodiark?). The Ascians refer to the Umbral Eras as The Rejoining (though admittedly this could just be referring to Zodiark and not the Ascians themselves).
A related observation that stands on its own, regardless of Ascian Origins, it is a commonly held belief that the Dark Crystal we see in the 2.0 ending is a representation of Zodiark (either as an entity separate from Hydaelyn or in place of her). I don't think this follows anymore. Zodiark predates whatever event creates the duality (and changes the nature of the planet). So, Zodiark would not itself be aspected as dark.
I think it's more likely that Hydaelyn and this Dark Crystal represent the light and dark sides of Zodiark and it was this change in Zodiark that caused the duality in the first place (or "was" the very change itself depending on how core Zodiark his to defining the nature of the planet).
The biggest loose end here is how the primals fit into all this. Ramuh's speech suggest they also predate the duality. Did they undergo a change due to the duality? Did they have a more permanent existence before the creation of Hydeaelyn which may be the beating heart of the very aetherstrean they now exist in before being summoned? What about Gaius' assertion that the Twelve are just more primals (I'm not inclined to put heavy weight into Gaius' comments given his obvious bias).