Eventually, most D&D turns into some weird allegorical… thing… as heroes who had heretofore been content with smashing kobolds and giant rats graduate to smashing devils and yugoloths.
I design my campaigns with a lot of detail at the bottom (up until now, it’d been set entirely in a single mountain valley three days ride across) and a lot of detail at the top (gotta know to whom the local pastor turns when her church has been raided; gotta know who the locals are racist against; gotta know who built that abandoned castle filled with coins of an ancient mint). I don’t usually bother with planar structure because it matters so little in game: Plane Shift may get you home or to the adventure site or to Sigil, but the planes are too big and I generally out-lore my players, so they’re not going to throw me too many curveballs.
I ensure that I out-lore and out-curveball them by changing everything.
So! My current campaign uses the 4e cosmology, mostly. I say mostly because I’ve eliminated the astral sea and elemental chaos as things, and what would have been astral dominions are converted into alternate worlds.
I like the idea of everything being survivably PC oriented because at a certain level, we’re all just people.
There’s the world you have been adventuring in, up until now. Let us call this The Prime Plane (or “The World”, though as you’ll see his would get confusing). Mostly like our world. Attached to it, like shadows or echoes, are the Planes of Dreams and Death; less scholarly characters don’t like to say “plane” and so are more likely to call the former The Dreaming and the latter… Well, they try not to name it at all, though the names The Grey Waste or Annuvin might get used. Collectively they can be called Echo Planes or Shadow Planes, though Shadow tends to be used as a euphemism for Death, and so may be contextually unclear.
Both of these planes, unalike though they are, share certain attributes. They have shallows, which lie near the Prime and overlap it and each other. Some times, in some places, they become so close they touch; when this happens, bleed can occur and elements from one plane transition and affect the other. The reason that this can happen is that the two Echo Planes contain rough analogs of the Prime; where a mountain lies in one it likely lies in the other.
There is no such thing as the Feywild. One familiar with that concept might recognize The Dreaming, though. This is the plane from which elves and fairies hail, and is reachable by madmen, children and poets. It’s a primeval place. It’s a duplicitous place. The land is alive, the animals can talk, and nearly everything is enchanted. Emotion and sensation are enhanced. Borders are fuzzy; one can slip into the Dreaming quite easily.
As an example of how easy it is to slip into Dreaming, mortals do it every night. They weave illusions for themselves, some of which escape and return to the plane from which they sprang; the sleeper awakens.
By traveling into deep areas of The Dreaming, without reference to analogous landmarks, the traveler may reach the Deep Dreaming and, from there, travel to any world which contacts the Dreaming.
There is no such thing as The Shadowfell. However, there is a land of the dead. It’s bad luck to say its name, so it has many: Hades, The Grey, Shadow, The Dark Realm. It, too, is haunted by spirits: those of the dead. Rage, grief and fear tend to rule here; at best, resignation and acceptance.
In death, all mortals pass through here. Given time, they erode into this place, passing beyond.
Devils and angels are both trying to use the souls to build their respective kingdoms; the difference is that the angels allow compassion for their charges to hinder them.
It is harder to slip into The Grey; death or a place of death will do it, and deep tombs or dungeons frequently begin to bleed through, leaving the barrier between life and death quite thin.
By traveling ever deeper into The Grey, a traveler will eventually pass through to the farthest shore, reaching another world which touches upon The Grey.
Strung between The Dreaming and The Grey are hundreds of worlds in an ever-shifting lattice. The Prime is one of them; others include:
Avernus, gateway to the city-world of Dis, of the nine worlds of Baator.
Celestial, homes of the Iluvatar.
Hestavar, a “completed” garden world.
The Elysian Fields, a shelter for weary spirits.
Arvandor, home of the Elves.
Pluton, the world of the dead.
Carceri, the prison planet.
Hoth, the frozen world.
Some are closer to The Dreaming, some to The Grey. In fact, most are unreachable from one, the other or both of these echo planes; the Prime is relatively unusual in that it touches both.
Anyway, there are hundreds of worlds.
Each world and its echoes are like swimmers in a vast sea at night. The swimmers are the Prime and her two companions, Death and Dream.
Below the surface are untold fathoms of dark sea, which is called The Abyss. Stacked behind every world are a thousand-thousand what-ifs and might-have-beens and never-weres. These are not questions of history and human choice, but rather the branching questions proffered by alternate initial conditions. These worlds form the Abyss underneath, spiraling away into Oblivion, the incoherent inviolate center of the Abyss. Because the set of never-weres is larger than the set of what-is, and two what-ises share all-but-one of their never-weres, beyond a certain shallow depth, all the abyss is a singular trunk of Abyss, uniting all of reality in entropy.
It is not just history which is denied in the Abyss; it is causality. As one descends into the Abyss one passes through impossible planes built along laws rejected during the dawn of time. Things lose their meaning: fire freezes instead of burning, rock floats instead of falling, lightning infects and lingers. Going deeper, other laws are perverted: wombs birth monsters, flesh twists, love is hate.
Worse: using the sea metaphor, our swimmers are drowning, fitfully sinking into the depths. The creatures of the Abyss — demons — seek to claw the swimmers down, hoping thereby to forestall their own Oblivion. These parasitic worlds and their denizens are harbingers of chaos and dissolution.
Unfortunately, the Abyss is convenient.
Effects like Etherealness use the Abyss because its shallows are unoccupied (usually). Even the early depths generally just make matter cease to interact, allowing passage through walls.
Effects like teleportation use it because it is easy to sink into the abyss and easy to conjure, albeit the plane resists attempts to dredge up its deep content, as they must be translated.
One final note: the Modron. These are constructs which enforce The Charter and hunt for violators of Oblivion. They hail from the Astral Temple, a “place” asymptotically close to being outside time. There, they build and maintain the god-machine which monitors/enforces, well, causality and all the other Laws. Their god-machine deals brutally with most interlopers, though its mechanisms are blind to bugs (Slaad) and violations of Oblivion. The Modron share this flaw, often needing to work through agents when their banes are involved.
Angels come from the Celestial worlds, though can be found in The Grey. They winnow the dead for the worthy.
Devils come from the Baatorian worlds, though can be found in The Grey. They winnow the dead for the valuable.
Elementals are from the natural world.
Demons haunt the Abyss trying to hasten each world’s slide into Oblivion. Their own relationship with the depths is propitiatory.
Aberrations are entities which violated Oblivion and yet return.
Fey haunt The Dreaming, though they particularly favor the world of Arvandor.
Undead haunt The Grey, though they particularly favor Pluton. They are also associated with the Abyss; the mindset that pursues eternal life continues to do so against the backdrop of Oblivion.
Modrons are goofy.