Okay, time to do some writing.

Re: Avatars. I think Avatars should stay mostly the same. Avatars are not simply relics; they are extra bodies for your god. With the introduction of Abilities constraining the forms your god can personally take, Avatars will be even more important. We could tie Avatars even more closely with the Abilities somehow, such as by giving Avatars access to the Abilities system.

Re: Spheres. Okay, this one's a biggie. I may have gotten a little carried away.

[b]Point 1, Topology of the Universe[/b]

tldr: Distinctions between Upper/Lower/Middle etc are only meaningful if the allowed direct connections between Spheres are restricted.

My personal preference is option D : There is no formal 'Upper' and 'Lower', with us instead characterising the Spheres purely by their connections and providing no cosmological constraints to how they have to be connected. Then the whole categorisation problem goes away because there are no arbitrary categories.

I'm getting the impression that most people are rather keen on keeping these categories, but the idea of connectivity being the primary feature relating the Spheres is still pivotal. If we can define how the Spheres are (or can be) connected, that is, if we define the topology of our universe, we can work from there.

First, I should define what I mean by a connection. A connection is any physical link between two Spheres. It could be a portal you could travel through. It could be being able to see a feature of one Sphere from another. It could be the border between two Spheres (which is especially relevant if you insist on the Spheres existing in three-dimensional space only). If there is no connection, the two Spheres have no direct interactions and cannot be seen or travelled between directly.

Consider the sample cosmology below, in which we have Galbar in the middle, the Core and Barrier below and above, and three (nominally) Upper and Lower Spheres. (I have drawn them as planes for simplicity. Wrap them around in your head if you want to get back to Spheres, but we're working in higher-dimensional space so drawing them as Spheres will get problematic once I start connecting them.) Note that nothing can get to the other side of the Barrier and Core without first passing through them. At the moment, none of the Spheres are connected. The topology is such that I can rearrange Galbar and the other Spheres freely, making the categorisation of Upper and Lower arbitrary. (There are other possible arrangements which I shall explore later.)

[hider=Unlinked Cosmology]
[img]https://img.roleplayerguild.com/prod/users/e46cbbb8-8346-46be-bef3-1f26701560b9.png[/img]
[/hider]

Now consider the next example. If we want a strict hierarchy of Spheres, we will have a connectivity where each Sphere links to the adjacent Spheres. Note, however, that one could equally define 'adjacent' as what Spheres a Sphere is linked to, rather than by arbitrary positioning in some kind of hierarchy. If we wanted to cement the distinction between Upper and Lower, we could have U3 (the uppermost Sphere) connect the the Barrier and L3 (the lowermost Sphere) connect to the Core, otherwise from a cosmological perspective there is nothing really stopping us from flipping the Upper and Lower Spheres around. We might decide that such a linkage occurs by default, or we could make it such that each link needs to be created. It appears people have been leaning towards the latter.

[hider=Hierarchical Linkage]
[img]https://img.roleplayerguild.com/prod/users/82ede5fd-971f-4868-a219-031ec7b32cf9.png[/img]
[/hider]

However, if people are to create their own connections, they would most likely want to connect directly to Galbar where all the action is happening. This will give an initial connectivity like the one below.

[hider=Direct Linkage]
[img]https://img.roleplayerguild.com/prod/users/c6573ae9-e7f2-47ae-b463-7da2e5912983.png[/img]
[/hider]

However, in such a linkage scheme there is no hierarchy or strict distinction between Upper and Lower. An equivalent way of expressing the above linkage is in the scheme below, where there are no distinct layers. If some layers started connecting to the Barrier and some the Core, then the Upper and Lower distinction could be used on them, but why would anyone want to make a portal there?

[hider=Direct Linkage, No Ordering]
[img]https://img.roleplayerguild.com/prod/users/45779b85-6b3a-4d4b-8499-f28fe180976d.png[/img]
[/hider]

Under natural play, if we don't constrain which Spheres can connect to what, we'll get something which looks like a bunch of organic connections without strict order, as below. This is fine. There will likely remain a thematic upper/lower distinction, primarily on where the connections are placed on Galbar, but it won't be strict. Middle Spheres are just as plausible as Upper and Lower.

[hider=Example of Natural Unrestricted Connections]
[img]https://img.roleplayerguild.com/prod/users/5d4d0aa3-e889-406a-b970-c5f53c63d3e4.png[/img]
[/hider]

But how does this tie in to the options? Well, the different options have different topologies for the Spheres. The examples I have presented above fit in with my option D with no strict Upper/Lower categorisations.

If we want an ultra-strict hierarchy, like the Greek celestial spheres model, then each Sphere would be represented not as a mobile blob in my abstractions but as complete layers similar to the Core and Barrier, as illustrated below. It is not possible to connect to another Sphere without passing through the intervening Spheres. This option has the downside of there being no flexibility in the nature of the connections.

[hider=Option: Strict Hierarchy]
[img]https://img.roleplayerguild.com/prod/users/76caf97e-3cc9-459f-9288-f6903ba44bfc.png[/img]
[/hider]

I have not seen people advocate for this option specifically. One strongly advocated similar option is Option B, having a strict demarcation between Upper and Lower. This can be achieved by making Galbar the border and having it such that Upper Spheres can link to Galbar and each other, and Lower Sphere can link to Galbar and each other, but Upper and Lower Spheres [i]cannot[/i] directly link. I illustrate this set-up below. If you drop this restriction, then there is no distinction between Upper and Lower except for in name, and a distinction only in name is a meaningless distinction.

[hider=Option: Strict Upper/Lower]
[img]https://img.roleplayerguild.com/prod/users/d894ef7c-b1b6-4154-99a5-eaca5c962df3.png[/img]
[/hider]

But what about option A, where we allow Middle Spheres? There are two possible topologies which allow this. One, illustrated below, is for the Upper, Lower and Middle Spheres to be separate categories, with connections allowed within the categories and with Galbar but not with the other categories. The Upper Spheres can also connect to the Barrier and the Lower Spheres can also connect to the Core.

[hider=Option: Middle Spheres are Separate]
[img]https://img.roleplayerguild.com/prod/users/333073c9-0e8d-43f8-a25f-e03f51217c9f.png[/img]
[/hider]

However, this isolates the Middle Spheres in a manner one might consider strange for a cosmology meant to be built upon a concentric sphere model. An alternative is to have the Upper and Lower Spheres separated by the Middle Spheres, of which Galbar is the most important. This cannot be represented with a two-dimensional topology, but I make a representation below anyway. The Upper Spheres can connect to the Barrier and the Middle Spheres (including Galbar). The Lower Spheres can connect to the Core and the Middle Spheres. The Middle Spheres can connect to Galbar, the Upper Spheres and the Lower Spheres. This gives us good behaviour for the connectivity of the Middle Spheres, but it makes Galbar less special by making it essentially a glorified Middle Sphere. Due to enhanced connectivity, people might favour Middle Spheres to other Spheres.

[hider=Option: Middle Spheres are Galbar-like]
[img]https://img.roleplayerguild.com/prod/users/be2c4483-2897-435e-a641-bc512e7e4a44.png[/img]
[/hider]

Option C instead considers subdividing the Upper/Lower categories into those which are close and distant from Galbar. In this topology model, these extra layers can be modelled by having the inner Spheres as a barrier between Galbar and the outer Spheres. So the Upper Outer Spheres can connect to the Barrier and the Upper Inner Spheres, the Upper Inner Spheres can connect to the Upper Outer Spheres and Galbar, Galbar can connect to the Upper Inner Spheres and the Lower Inner Spheres, the Lower Inner Spheres can connect to Galbar and the Lower Outer Spheres, and the Lower Outer Spheres can connect to the Lower Inner Spheres and the Core. This is more simply seen below.

[hider=Option: Upper/Lower Inner/Outer]
[img]https://img.roleplayerguild.com/prod/users/aa4608d1-787e-463b-882a-183896ee4e49.png[/img]
[/hider]

So, that's a lot to take in, so let me summarise below. I have 6 options: Unrestricted Connections; Strict Hierarchy; Upper/Lower; Separated Middle; Galbaric Middle; Upper/Lower Inner/Outer.

[hider=Summary of Cosmology Options]
[img]https://img.roleplayerguild.com/prod/users/2aee37c5-a02b-4449-995f-30a0dc4c82f9.png[/img]
[/hider]

My preference is for either Unrestricted Connections or Upper/Lower. Strict Hierarchy is too restrictive and has the complication of everyone having to decide their position relative to everyone else and is broken if any new Spheres are created. Separate Middle and Galbaric Middle feel rather odd in their implementation of Middle Spheres, although maybe some will like it. Upper/Lower Inner/Outer feels like too much added complexity; if a god wants an Inner Sphere, they can connect to Galbar; if they want an Outer Sphere, they can opt not to connect directly to Galbar. It's up to them.

I'll repeat that categorising the Spheres [i]a priori[/i] as we are currently trying to do is only meaningful if there is a physical difference between them. The main physical difference we can control without dictating the resultant pantheon and physics too much is controlling the connectivity between Spheres.

[b]Point 2, Connectivity and Access[/b]

The portals connecting the Spheres are very important, as they define travel between the Spheres. But the portals are also created by players. Therefore, subject to the constraints we put above, the nature of the connections should be up pretty much entirely to the player creating the connection. The Portal need not be a discrete location.

Consider the hypothetical god of dirt, who wants the Sphere of Dirt to be accessible by digging in the dirt. The Sphere of Dirt is not automatically accessible from Galbar. It influences the dirt to make it fertile and whatnot and it creates dirt on Galbar, but it is not physically accessible unless the god of dirt creates a Portal. In this case, the Portal is a region of dirt, and the condition of access is digging into it. We might rule that having 'all dirt' is too broad to be a Portal, but you could make it a specific region of dirt, perhaps under certain sacred hills. If we insist on Portals out of Galbar being difficult to traverse, then we might attach extra conditions, or make it necessary to dig really deep.

Now consider a hypothetical god of sky, who wants the Sphere of Sky to border Galbar along the whole sky, so anyone who flies up high enough enters the Sphere of Sky. They could put their Portal as the whole sky. If we want to impose a 'difficulty' condition, then there can be storms and wind which makes flight difficult.

The god of the wilderness might like the have forests connect to his Sphere. As previously suggested, the Portals might be very difficult to find reliable, but it is quite possible to just stumble across them without noticing. Perhaps the exact location of the cross-over between Galbar and the Sphere varies, although is in random locations deep in wild forests. The god of the wilderness could control where the Portals appear (possibly as an Ability) because he is attuned to his Sphere and the Portal's location is rather nebulous.

These points indicate that a Portal need not be a discrete location, but is instead any connection between two Spheres of any form.

Another customisable feature should be the ease of access and strength of connection. A god may or may not want a strong connection between Galbar, so should be free to choose. A god might also like an easy or difficult connection. The benefit of an easy connection is that allies can readily access your Sphere and any facilities within. The benefit of a difficult connection is security against monsters, mortals and enemy deities. If you want to reinforce difficult travel, you could specify that a Portal between Galbar and another Sphere [i]must[/i] be difficult to traverse, if not by design, then by the pressure of the Sphere's influence flooding downwards. [i]How[/i] the traverse is difficult is up to the player. Although I'd argue that we need not be too stringent here.

Also customisable is how it connects. The connection would have to be extra-dimensional (read: magical), otherwise the Spheres aren't special planes of existence but just regions on a planet. That said, if a god wants, they could have what essentially amounts to a physical border with another Sphere, but that would be chosen when the Portal is created.
 
 
Now, I've been writing this post all day and I'm out of comments on these matters for now. I've laid bare my thought processes. Let me know what you think.