Going out on a limb here... the first two "gods" we know of are Anu and Padomay. What if they created the first world (kalpa), and each subsequent generation had more? This kalpa has 8 divines, and the next will have 16 or 17 (or 12) depending on questions on who counts as a Daedra and who gets to come along and whatnot. Oblivion would then be populated by various spirits, some old and some new, and a few mortals may ascend as well and BAM we have even more Aedra for the next Kalpa! Follow me?
Also, since when was Dagon connected with Magnus? I haven't been around the Lore forums as much lately, did I miss something?
I like to imagine it has more to do with the "offspring" of Anu and Padomay... otherwise, the Altmer are going to have a [NUMINIT]-ton of universes to destroy in their climb back up the Divine Escalator (which, as everybody knows, is permanently jammed in the "Down" direction, moving at "Holy Butt That's Fast" speed).
So Akatosh(/Alduin/Auriel/Tall Papa/Big Daddy Dragon) is the first spirit to have formed (that we know to have survived), and makes it his job to eat all of the kalpas, for one reason or another (maybe Divine Tuesday is "kalpa night", who knows). Perhaps there have been other kalpa-eaters, but we only know of BDD. I'm pretty sure, once BDD (or, linear time/action-response) has set himself up, other spirits find it both easier to survive, and that they no longer can do everything(/nothing) at once. From THAT, I personally believe, is where kalpas and worldmaking and Padomaic awesomeness can really start to shine. Spirits being tricked/persuaded into giving of themselves, only to be devoured/replaced, for the cycle to repeat itself. We don't know if the "new gods" are copies of the old ones (which is, it would seem, implied by the whole Anticipation reference), or how that works.
Although the Peryite thing throws a massive wrench in my thoughts. Is he the new Alduin, too? Or just an Akatosh? Does that mean he becomes part of Alduin (or even just a new point of view), or are we to find out that they, somewhere down the line, became completely separate individuals, completely against most of what I thought I've read, and the Omnipotential lesson on Aedric thought processes found in these letters.
I'm uh... just gonna go wait in a corner until proweler, Lady Nerevar, or MW show up and explain it in a way that makes sense.