Dragging a quote from Moose over this way, since I think it's high time for a lively discussion of high-fantasy metaphysics.
I suspected something a little different, and still do, though the "World of Darkness" arc having little in the way to do with much of anything on the table has, perhaps, weakened this position somewhat. I imagine that Hydaelyn is the world of "light" split off from some original dualistic world by some being/beings (the Twelve?) powerful enough to alter reality itself to suit their ideals, and that what they left behind was the world of "dark." Perhaps the "void" the Allagans called the "World of Darkness" is, in fact, the space between these two worlds.
We know that some of the Ascians are of "our" world, but that in turn can be taken to mean that some aren't. There may be another world out there, not unlike our own, that exists as a separate "layer" with the same surface area, but different geography. Is that world Zodiark? Is it dying and/or dead? Is recombination with Hydaelyn the way for that world, or both worlds, to be saved? "Rejoining" two geographically distinct worlds will, of course, have "calamitous" repercussions in the short term, but is perhaps necessary for both of them, for reasons known at least to Eldibus (and now Urianger, perhaps). It may be that the alterations to the landscape after each Calamity are, in truth, pieces of that other world being merged onto our own.
I wonder if it was really Ultima used on the Isle of Val, and not simply a large-scale X-Zone-type spell. ("Banish" appears to be a fluid-like damage spell in XIV, so I'll use the old name for the classic "send object elsewhere without passing 'Go' and collecting $200.") Perhaps having the Students turn out to be alive on that "other" world would be too "easy" a direction to go with that storyline, but our current perspective is rather appallingly limited.
The necessity of light and darkness staying in balance for the world to survive would also be almost too easy a punchline, but we already had hints towards a sort of "flood of light" in the years leading up to the Fourth Umbral Era (rather, one hint I desperately and irrationally cling to), and another resulting from the overuse of White and Black Magic. If the Ascians were involved in all of the Calamities, they may have some greater reason to keep the light from shining too brightly. I wonder sometimes if we have it all backwards, and the light is, or at least was, winning overwhelmingly and moving towards decisive victory in a field of battle we still lack the viewpoint to see.
But we lack so much information that none of us can really guess correctly, so I'm also curious what the other folks around here can come up with.