In the setting I'm writing, I'm using basically a 4e basis for the cosmology.
We have the prime material with its echos Shadow and Fey (or is the Prime an echo?). The "near" Astral is called the "Ethereal"; the Astral Plane has at least 5 dimensions of space, and when you approach a plane you are in the Ethereal of that plane. The Fey and Shadow are also nearby the Prime plane in the Astral, so the Ethereal of the 3 overlap (imperfectly).
The elemental chaos is a place, because that is cool, and it lets me have hybrid elemental creatures. The abyss being an pit within the elemental chaos is very evocative, and I love the idea that a shard of pure evil generates the entire abyss.
My alignments are simplified:
Lawful is organization and abstraction over individual connections.
Chaos is individual connections over organization and abstractions.
The Law:Chaos axis is about how much abstract ideals vs personal connection impacts your values.
"I follow the crown, not the king" is a Lawful ethos, while "I follow the king, not the crown" is a Chaotic ethos.
Good is self sacrifice for the benefit of others.
Evil is harming others for your own benefit.
The Good:Evil axis is about how wide you create your in-group, and how much you value the out-group. If your in-group (thing whose benefit you value) is yourself only, that is more evil. If you extend it to your family, or your friends, or your clan, or your people, etc this moves you further towards Good, as does as your "discount factor" for benefit to others in your in-group compared to yourself shrinks.
From this, you can see how a given religion that is any of these alignments could be sensible, and not "I'm a deranged psychopath to believe in this".
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The outer planes are indeed the places where the Celestials and Gods lived, but my cosmology is post-apocalyptic. The great war against the adversary was fought, and the Gods won a Pyrrhic victory. The adversary was contained, most of the Gods where slain, and the remainder have since died of their injuries. The outer planes are a complete mess; the angels have been slowly going insane for millenia. Asmodeus led refugee angels to the 9 Hells and forged them into Devils in an attempt to survive, preying on the faith of mortals.
The various churches on the prime material plane are fueled by divine relics - pieces of gods and celestials with divine energy in them. They commune (with increasing difficulty) with insane angels left in the outer planes, getting cryptic answers back on their Divinations, as the angels are unable to say "your god is dead". High level PC-type beings with planar travel are nearly non-existent, barring the a lich or similar.
They bless paladins using said relics (which imbues them with a spark of divine energy); the orders of paladins are very selective about who they give such blessings to, as you can't take the spark back short of killing the paladin. There are, naturally, other ways to become a paladin, as there are both lost relics and loose divine energy in the mortal plane that could imbue a person with becoming a paladin (or cleric).
Most divine magic is manipulating said divine relics, or using them to create blessings that ordaned members can then use later; think "can read a divine scroll" as a NPC-priest class ability, and sequestered NPC-monks that spend their days scribing such scrolls as an institution of organized churches. (The ink might be infused with the tears of an idol of their god); "divine magic-tek" becomes efficiently using the known relics power to produce magical effects.
Arcane magic isn't in a much better state. To produce effects like PC spells, NPC mages read scrolls; they have a chance for the scroll to not self-destruct, and it drains them (uses their spell slot) as well as risking the scroll. The scrolls are created by clerks using inks and materials that are magical in nature (harvesting from the world, instead of from divine relics) and each scroll is unique (as the clerk has to win a fight against reality in order to imbue the scroll with the spell).
Pact magic is the other form of magic - making a deal with a spirit or other being. Rangers, Druids and Warlocks all use pact magic. Here, the supernatural being is the one "actually" providing the power. For the Warlock, a shard of power is embedded in their soul; Eldrich Blast is focusing raw soul-energy leaking from the wound. For Rangers and Druids, spells are actually actions done by nature spirits.
Sorcerers are very rare. Bards who can do more than hedge magic (cantrips) are nearly unheard of.
My Artificers are Gadgeteers. All of their "instant" effects are rewritten to be stuff they build and prepare over short rests (even PCs). NPCs usually take even longer to build their gadgets. Making a gadget that a non-artificer can use is a very long process.
PCs are all prodigies; the rules work as written. A PC wizard is like an NPC mage who can actually hold a spell in their head, nearly unheard of, and cast it without a scroll. Similarly for clerics, able to do magic without using divine energy from a relic?
Back to the cosmology, this means that when the PCs get to the outer planes, I'll have to create a horror-show version of them. Insane angels, feral celestial creatures, etc. In theory the PCs can bring along divine energy and rebuild parts of them.