Thanks for that. There's still some gaps in that story that bug me though.
(I'll start using Jaamuran as the name for the solar system, but I don't think that works for the planet. So, earth for now.)
The one that's bugged me the longest: the necessity-or-not of a sun. In the earth's early going, there was no sun. Toteth said he observed other systems, got inspired (vol 3, pg 39), and created Vona based on those observations. (Really: why isn't Toteth worshipped a god? He is the sungiver, for crying out loud!) Anyway. So in your chronology above, during steps 1-4, the earth would look at lot like Draenor (the land "beyond the dark portal" in the Warcraft universe). Then during step 5, the biggest change isn't that Mavisha breaks up the Yerasol continent, but that the sun comes out! That would dramatically change the world's ecosystem (down with the giant mushrooms, up with the trees!). Who knows?, if draenor-earth had polar icecaps, and the sun melted them, that could explain the sudden sea-level rise.
But wait, when the party rejoins reality at the beginning of adv13, the world is dying from a lack of sun. Is it really "dying", or is it just finding a new equilibrium and reverting to back to its "looking like Draenor" days? (Setting aside the fact that the Gyre will crush the earth before the ecological shift can finish.) But wait again! If the party chooses the "rejection" ending (as RW's homegroup did) .... how does that not result in icyends?
In my head-canon, the planar-nursery you refer to in step 1 is a nebula, and this gives off a warm, life-giving glow. It's not as bright or hot as a sun, and it surrounds the earth (i.e. no real difference between day and night, per Toteth's "monsters" comment (vol 3, pg 39)). It also kinda conceals the earth, as a nebula is effectively in space-fog. That's why earth's not swarmed instantly by a million different raiders, but it takes a bit of time for the Golden Legion, Gidim, etc., to find it. But the space-fog (nebula) doesn't actually keep anyone out ... it's concealment, not cover, ya know? So once earth gets kicked out of the nebula, it needs something like a sun to survive, or else it will freeze, a la adv13. And to tie it all together: what's happening in the "rejection" ending, is the planet is going home: returning to the nebula. In that scenario, the text merely says "a purple glow like a dawn rises" (vol 3, pg 233) but never actually finishes the thought.
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So, as I move slightly closer to the OP topic, I want to ask something about the planar realignment caused by The Grand Design. Is there a Watsonian explanation for why the Av-Ziggurat linked up with Jiese? (You gave the Doylist one above.) Because in the Jaamuran system, Av was in the "moon slot", and fulfilling the role of "life" planet. In the Grand Design, Mojang is in the moon slot (vol 3, pg 225), filling the "life" role. So the Av-Ziggurat should corresponding point there. Jiese was left as the "fire" planet in both old and new configurations.
The only other reconfiguration mentioned in the text hold together: in the Jaamuran system, the Apet-Ziggurat was linked up with Apet, which was in the "space" slot. Whereas in the Grand Design, "space" is filled primarily with Fourmyle (with Baden riding shotgun) (vol 3, pg 225 again). And now the Apet-Ziggurat is linked with Baden (vol 3, pg 39). So that lines up.
So the Jiese-Ziggurat in Crisillyir should stay linked to Jiese, as it's literally the only that that didn't change between the Jaamuran and Grand Design configurations.
On that topic: the Ziggurats seem to somewhat impact the environment around them. Mavisha is in water, Avinola is in air, and Jiese is in a desert. Given that your new setting book takes place 20 years in the future, would the Antwalk Thicket have started to turn desert-y, or at least become a lot lot hotter? (Due to the power of Jiese leaking out, as opposed to Av, which made sense for a "thicket".)
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OK, actually back to the OP: my question was "what is immediately on the other side of the Ziggurat portal?" Clearly, it's not another Ziggurat, as earth-dwelling orcs built those. But if you were a native of, say, Jiese, and you looked at the "spot" that was linked, what would you see in that spot? A flickering portal in the sky? Or did the Salamanders build their own Ziggurat-analogue, to protect against invaders from earth?
And special-case for Av: if the players were in the Dreaming, and went over to the Antwalk Thicket, and to the place of the Av-Ziggurat, what would they see? And could the fey "get through" from their side? (I'm thinking of a spurned lover of Rackus or somesuch.) The barrier between Av and earth is quite a bit more porous than the other planar boundaries afterall.
And this post has become overlong. sigh