@BBeastI think a big theme in your post, and one that I think is misguided, is to see the difference between upper and lower as a physical difference. It is a thematic difference, and I think treating each sphere as a formless blob with no content is detrimental.
Take magic.
Celestial magical sphere: Linked to stars and the sky, maybe be related to the movement of the stars, the time of the day, definitely related to the "star" that is how Galbarian mortals see the sphere.
Chthonic magical sphere: Related to earth, likely more telluric in nature, if a mage in the former case would raise his hand to reach for the magical realm, this one needs to connect with that which is bellow, walking bare-feet, channeling magic through the body, whatever.
Take soul
Celestial soul sphere: Probably Heaven or Valhalla like, angels and Valkyries, sky burials and funeral pyres
Chthonic soul sphere: Hades for example, boatman and reapers, crypts and graves.
There is a big change, and that goes beyond portals and physical place.
@CycloneOne thing I expected in the docs, but couldn't really find, is a model of what you are thinking. With examples, models, something like a few example spheres and how they interact. It clearly isn't the Aristotlean model what we are doing here, there is a weird mix of Aristotles like layer spheres and orb spheres, and each time these things are discussed, I go out of it knowing less and less, with mixed messages like "There are no exceptions except the exceptions"
The disadvantage of the Galbaric Middle is that it introduces significantly more complexity to an already potentially confusing system. Having a divide between the Upper/Lower gods along the lines of the Aesir and Vanir, Aedra/Daedra, or any other example was a topic I found intriguing and was a primary reason behind me conceptualizing the distinction. Where would the middle gods under Galbaric Middle fit into this? Would they be neutral and of neither side? That might make the whole thing less thematic, but then again, I expect that some gods won't really see the universe in such a divisive way and won't care so much about exactly where any other given god lives so much as what said other god does.
It is better to have a complex system that deals with its issues than a nebulous unclear system. I think if you want a strong thematic difference between Celestial and Chthonic, you need to at least state that and what you overall view on what is Upper and what is lower. Once the themes are put on the table, we can discuss them properly.
Speaking of which, I still think land gods are a problem, from what I see, the Chthonic realm is way too limited, and throwing trees to the sky because of canopies is... eh. I don't know crap about the nordic myths since I found them boring, but in terms of Aedra/Deadra, all Aedra would be Celestial, and... most of the Deadra as well, since they are outer gods, Azura, Clavicus, Hircine, Vaermina.
It also makes the metaphysical maps of the world weird, with the realm of rives, trees and the fairyland all side by the side with sun, with the stars. It makes sense in a world like Tamriel where all worlds are outer worlds, but is this what we are going for here? Weren't we going with Aristotlean layers, with a realm of wind, a realm of fire?
We need to have a guide, a base, we can't have cake if the invitation says "bring ingredients for pastry, maybe?"
Then we can decide what to do about connections, if we are to have a separate mini realm of the skies or have it be the layer which we call the sky. What to do about land gods, what differs the underworld from the upperworlds.
Lord Zee
Ashh
Frettzo
Vec