What's Your Cosmology?

Well, despite my efforts, I have yet to successfully put together a Planescape campaign lasting any notable length of time. Nevertheless, I have extensively thought about the details of the setting, at least when my constantly changing interests and obsessive personality return my attention to Planescape, as occasionally occurs. Thus, I have developed a pretty definitive idea of my version of the Planescape cosmology. It is pretty close to the 2e sourcebooks, because I prefer to follow canon when possible, but I have also made many personal decisions about aspects of the setting not covered in the source books. Here are just a few of those idiosyncrasies, particularly in respect to the subjects you listed above.

• I use the Great Wheel with all of its paths (Styx, Oceanus, Yggdrasil, Olympus, Infinite Staircase, etc.) as they are described in 2e sources.
• I consider the planes, as well as every layer thereof (when applicable), to be three-dimensionally infinite, though separate from each other.
• I think that Outer Planes are not mappable, as while the sites therein tend to remain stable due to common knowledge / belief in them, the paths between them are more mutable, changing in accordance to the beliefs and actions of the beings currently traveling through the particular area. Thus, relation of the sites to each other is subject to change in both direction and distance, though some constellations of sites are slow to shift.
• The Inner Planes are more constant, if no easier to map due to large expanses of elemental substances undistinguishable by anyone other than natives. Despite the Inner Planes being infinite in all three dimensions, natives thereof are able to add extra dimensions to their travels that allows them to approach an ‘adjacent’ Inner Plane and even lead unwitting visitors in a particular direction on those extra dimensions. These extra dimensions form the ‘sphere’ of Inner Planes that is commonly depicted to represent the connections amongst the otherwise infinite planes.
• I’ve not considered the nature of the Material Plane too much, as I’ve not read much on the Spelljammer setting. I do not know whether the Crystal Spheres are infinite in volume within a greater infinite sea of phlogiston or if they are finite solar systems within the deep space of phlogiston.
• Transitive Planes are much like they were in 2e, with the possible addition of the Ordial. The Astral does not reach the Inner Planes and does not have demiplanes within it while the Ethereal does not reach the Outer Planes and has a shallow and a deep with numerous demiplanes within the latter. Shadow is just a huge demiplane.
• On the Outer Planes, the realms of powers are like any other notable sites floating among the infinite expanses of their respective layers’ indefinite hinterlands. The difference, however, is that they are subject to the whim of the power to whom they belong, and can thus appear different from within than they do from without. They do not necessarily follow the physical laws of the surrounding plane. For example, it might only be a day’s travel to walk completely around a particular power’s realm, but it might take months of uninterrupted travel to walk directly through it. The inside dimensions do not have to fit within its outer borders. Powers exist throughout the Outer Planes, Baator included, but only the more well known powers are likely to be encountered by accident while traveling through the infinite and mutable expanses of the planes.
• While powers have ultimate power within their realms, they rarely travel outside them. The exemplars and other outer planar creatures rule the non-deific sites and the hinterlands around them. While even a lesser power would be able to take on an entire army of outsiders on neutral ground, such armies are inexhaustible, and such fighting will eventually wear down the faith-based power of the belligerent deity. The loss of outsiders will be significant, yes, and it might affect that surrounding political landscape amongst the exemplars, but at what cost? Few powers would care to make the sacrifice. There are also all sorts of agreements between powers and powerful exemplars that keep things civil, whether by official contracts or simple mutual respect, depending on the side of the law/chaos spectrum is being discussed.
• Every Outer Plane has many different types of outsiders, both native, introduced, and created. Only one ‘race’ or subtype of outsiders, however, have an intrinsic connection to any one plane at any one time, and these are what I call exemplars. They are usually found on the more cardinal and semi-cardinal planes, but can develop on the eight intermittent planes as well. These are not necessarily constant throughout the ages, as is evidenced by the Ancient Baatorians being replaced by Baatezu and the Obyriths being replaced by Tanar’ri. It is also possible that when the outer planes were fewer in number and more encompassing in their essences, there were other greater exemplar races for each of the overarching alignments, the most notable being the Baernaloths.
• Yugoloths are an artificial race, composed, created, and/or evolved from various now-forgotten outsider races.
• Modrons are the exemplar race of Mechanus. However, after the whole Tenebrous debacle and the Harmonium induced appropriation of Menausus (and the now insane Formians found thereon), the Modrons have retreated to the innermost cogs of that plane, centered on Regulus. The Inevitables became more prominent to pick up slack for the Modron’s decreasing influence, but despite all the mysteries of their origin, they are not the exemplars of that plane.
• Rilmani are major players, but they tend to work subtly, using disguised infiltration and long cons to play the other forces to neutrality’s advantage. In all appearances, they seem to be xenophobic and uninvolved, and even those that know of their involvement cannot estimate its extent.
• Sigil is of an indeterminate and constantly changing size. Much like the outer planes, the better known areas of the city tend to be more stable than its back alleys, and the depths of Undersigil is unknown to anyone except, possibly, The Lady and her dabus (dabi?).
• Technological advancement in Planescape need not follow the pattern taken in our own history. Rather, the planes might have clockwork devices unimagined in our world while still lacking gunpowder or steam engines in any large numbers. Sigil, being the great mixing pot, has people that depend solely on technology, others that depend solely on magic, and many more inbetween.
• Psionics exists, but it is rare and less understood than magic. Certain races are more naturally intimated with psionics (Githyanki, Psurlons, Illithids, etc.), but almost any species has a chance of producing psions, though less commonly than more convention spellcasters.
• 99% of the people, even those living in Sigil and the Outer Planes, lead relatively stationary lives and have only vague notions about the planes beyond their home. Though news eventually reaches them, it is mixed with ample rumour. Planewalkers, the adventurers of the planes, and interplanar traders are the remaining 1% that travel enough to have a clear picture of the planes, and even they don't know all the darks that a regular DM would.

So, yeah, that’s enough for now. I’m tired of typing.

Mine are that the planes are generally more loosely defined in the ways they are. Other cosmologies can see the planes configured a certain way, and they'll be right about some things, but that leaves holes out there for the unexplainable and weird. The Great Wheel is most commonly used as it's the most robust cosmology that accounts for most of the multiverse, except for odd planes and places like the Far Realm.

The Ethereal Plane touches all planes and you can get around by going into the Deep Ethereal and emerging in the border Ethereal of another plane. The Plane of Shadow touches all planes and so does the Astral. There is a Deep Shadow Plane you can go to which connects to the Shadows of all non-transitive planes.

There also is the Plane of Mirrors, The Faerie World and the Spirit Realm, and the Planes of Dream. Along with many places that don't quite fit into the Great Wheel like some of the nine Lokas, some worlds described in Beyond Countless Doors, some of the places in the Eberron cosmology and so on.

The technology level around the multiverse is on average that of the 19th century there are some places that are more primitive, and some that are more advanced. The rules of physics and reality does change between planes, and technology works dependent on "Consensus" to borrow the term from Mage: the Ascension.

A world known as Earth is reachable in this cosmology, which resembles our world in that there's the internet, TV shows and global warming and nations like the USA, China, Russia and the UK, but beneath it all there looms beings of "the Shadow" elves, goblins, yuan-ti, fraal and others who live in a world ignorant of their existence and the fact that magic exists. There's a force out there called Dark Matter by the Hoffman Institute which was known as the "Red Death" in the past. And shadowy organizations such as the Illithids, the Illuminati, the Planar Trade Consortium, Knight Templars, Final Church, Hoffman Institute and more fight a secret battle.

Finally in some ways with how the inner planes are there is a place where they all meet which could resemble an Elemental Chaos, but this Elemental Chaos has nothing to do with Limbo or the Abyss.

Our cosmology is a mixture of PS/FR and homebrew. I have renamed some of the planes and layers, mostly those sounding Greco/Roman/Norse cause I don't like too many real world copies and analogies. We did the same with Toril.

The number of planes is unknown, and they aren't arranged according to any noticeable pattern. The interaction between the Astral and the Ethereal is the base of known existence and they encompass all believable/imaginable planes. Outside there are infinite Far Realms. The Prime Material doesn't consist of crystal spheres, but that doesn't mean that the physical laws are the same in every galaxy and solar system.

There are no Inner Planes, but the Elements permeate all existence. Examples: Fire & Metal have a large presence in Dis, Water in Abysm, Thalasia or Aquallor, Ice in Ocanthus, Negative in Thanatos, Salt & Crystal in Azzagrat, Wood in Yfel (renamed Niflheim) & Arvandor, Magma in Khalas, Metal in Mechanus, Mist in Belierin, Dust in Mithardir, Vacuum in the Prime, Temporal is mostly everywhere except in the Astral, etc. Only the Outlands have the equal measure of elements. Elemental Conduits are a nexus of planar pathways/connections, so Wood is like the World's Tree or Ygdrasil and can be used for example to travel from the jungles of Sseth to the great pine gates of The Death of Innocence. Many Conduits are warded. The Water Conduit is similar and on the nastier planes is like Styx, while on some better ones is like Oceanus except that each race/culture has its own name for it.

Known planes:

The Astral
The Ethereal

Concordant Domain of the Outlands

Clockwork Nirvana of Mechanus

Seven Heavens of Celestia + Abellio and Buxenus as ''rogue'' layers. Buxenus is coterminous with Mechanus.

Nine Hells of Baator

Xaos includes former Limbo, Abyss, Pandemonium (connected to the Far Realms through the Air Conduit), Unseelie's Court, Fury's Heart, Fated Depth's (where reality dissipates). e.g. domain: Fermata (Finder Wyvernspur, god, of cycles of existence), coterminous with Argentil/Playlife & Firelakes of Geneva/Dragon Eyrie (Garyx's domain)

Playlife layers: Arvandor, Annwn (coterminous with Grey Waste), Brux (triple-layer, coterminous with the Outlands, one part slipping to Shadowfel), Ladinion etc. domains: e.g. Argentil, Evergold, Land of the Hunt (part of Xaos) is coterminous with Arvandor and Ladinion, incursions of Malar, Svartalfheim (most ancient part of Arvanor, exiled drow now live here) ...

Grey Waste layers: Oinos, Yfel, Zlo

Soulhaven mix of Elysium, Bytopia, Green Fields, Dwarfhome (e.g. coterminous with Ladinion/Playlife, Nidavellir/Jotunheim, Solania/Celestia and Abellio/Celestia)

Blood Rift Plane of conflict, violently breaches all known planes, spilling war, more often on fiendish planes, Blood War site, most parts of Acheron, Clangor, Nishrek and Ysgard are incorporated here.

Dragon Eyrie Similar to Mount Olympus. Asgorath (Io) is an entity of Balance. Nine layers, e.g. Chasms of Greed: coterminous with Avernus/Baator, Tiamat, Xymor's Palace: coterminous with Mercuria/Celestia, Xymor (Bahamut), Firelakes of Geneva: coterminous with Xaos, Fire Conduit, Garyx ...

Faerie Parallel with the Prime material, ''eastern'' parts of Faerie (Shou Lung) are known as the Spirit World. Deep Faerie is an area not so parallel with the Prime, it is a ''world between worlds'', leads to all other ''sylvan'' outer planes.

Shadowfel Renamed Plane of Shadow, I like the name. Parallel with the Prime. Barrens of Doom & Despair is a part corrupted by Yfel/Grey Waste, otherwise is mostly unaligned.

Demiplanes, Quasiplanes and similar, examples:

Dreamworld Every dreamer creates his own plane, with skill it can merge with dreams of others, and can be coterminous with the Astral.

Dweomerheart Mystra's demiplane of magic. Important city of planar arcane trade (mercanes). Many portals, one big to the City of Judgement, one huge and several smaller to the Azuth's city in the caverns of Buxenus/Celestia. Temporal Conduit (regulated time travelling). Coterminous with Faerie, where the city extends (in fact this ward is the most ancient part of the city, the demiplane grew from there) and with Clockwork Nirvana, Dweomer-jag ward.

The Fugue Kelemvor's demiplane of the dead. Negative and Positive Conduit (used for repairs and cleaning due to raids). Coterminous with Oinos, souls are sometimes taken there, not known why.

Cynosure Now that Ao's dead, it's accessible to anyone. Gods still visit. Each god percieves it differently. Fights.

Khalas, Chamada, Mungoth, Krangath yugoloth demiplanes, artificially constructed for some experiments, probably Blood War, cause they are often seen in Blood Rift.

Jotunheim Once a full plane. Now in decline. e.g. domains: Alfheim (Arvandor/Playlife), Muspelheim (Blood Rift, Fire Conduit), Niflheim (Yfel/Grey Waste).

Deep Caverns Underdark demiplane, coterminous with Baator and the Far Realms. Illithids, beholders ...

Hammergrim Immense fortress of the duergar, coterminous with Blood Rift.

etc. would be too long

other questions:

Planes aren't full of power's realms, in best case gods can control a single domain or a demiplane, in Baator and the Waste there are no gods, tough some realms of the gods have connections there whether they want it or not, in Blood Rift some gods are minor mercenary powers like Red Knight, Uthgar's armies fight to stop war spilling to the North ...

Mechanus has inevitables, formians and modrons

the rilmani are subtle of course, tough there are exceptions

Sigil, 20 million

Tinker gnome items for now are exclusive in Lantan, and in Gond's demiplane

psionics is about 1000 times rarer than magic, but depends where

I include Eberron in the Great Wheel too, with the same gate-moon concepts. The Eberron planes match pretty exactly to various planes in the Planescape cosmology, so its pretty easy.

There's a couple of other local tweaks. In my cosmology, Olympus/Arvandor has twelve layers, not 3 (See my article on the Hidden Layers of Arborea for a description). I always thought that the CG 'corner' alignment got short-changed, with three layers, one of which was mostly useless.

The other tweak is that the Far Realm is actually sort-of connected to the Great Wheel. It always bothered me that WotC created this 'other' realm which messed up the neat cosmology that they'd created. More evil than evil? More bizarre than Chaos? I don't think so. More on this when I get that article finished

This is a world I have not yet tested. I'm submitting my ideas both to this (planescape) community and the spelljammer community. I've got a fairly detailed rationale for my modifications at my web site at http://www.squiz.btinternet.co.uk/cosmology.html.

I haven't yet decided on everything, but:

Player characters never have to declare their alignments - I decide their alignments based upon their role playing. But if they are serving a being or organization, they sometimes have to follow a set of rules.

No prime material plane - instead each prime world is simply a big demiplane.
No phlogiston - space ships travel through the deep etherial, which has ether flows instead of phlogiston flows.
Sigil is not on top of the spire - it is a demiplane.
The ethereal touches all outer, inner and demi planes.
The astral touches all outer, inner and demi planes.
The plane of shadow is the same place as the border etherial.
Dreams happen on the astral, not the etherial.
I intend to keep the Great Wheel and the other paths mostly the same.
Most other worlds including fairyland will fit somewhere within the great wheel.

It is plausible that the factions are a dominant force in Sigil but not throughout the rest of the infinite outer planes.

As for technology: The attraction of magic for me is that it makes it easier to fill the story with poetic balance. Any other technology in the game including psyonics and mechanicals should preferably serve the same function, and should certainly not hinder it. For the most part, technology is medieval.

Is the prime material considered one plane made up of crystal spheres and phlogistin (or whatever), or are prime worlds considered separate layers or planes reachable through wildspace?

The Prime is much like our own universe, except that most planets have some form of life on them and are usually around the size of Earth with air and similiar gravity. However some planets are "hidden" within demi-planes for whatever reason (protective gods usually) and need a Key to enter.

Are there independant Powers in Baator?

Independant? No, they are all under strict and binding contracts! Most gods would rather not live there and many have been relocated by my good self to other planes.

Do the inevitables run Mechanus, or the modrons? Is it infested with formians?

 After Primus died in the Orcus's rampage. The new Primus as worked to remove the taint of anything but Law from his plane. The modrons run the plane and now never leave lest it is of extreme importance. 

The inevitables are the ones that do work outside of the Clockwork Nivarna. 

Infest is a bit strong; on a infinite plane the formians are but drops of water in a ocean. They have been put to use, but generally ignored.

Are the rilmani a major player in the planar power balance? Do they openly take sides to restore the balance, or are they subtle?

The rilmani are nobodies, they are not angels of balance, just a very neautral race. They are not a major player anywhere because the idea that no side should win makes everyone losers. 

About how big and populous is Sigil?

Imagine wandering New York, without a map, that has sections of it altered every few years. It's bustling and busy.

Sure, most are raritys. That said Sigil has printing presses and gas lamps and various do-dads in some Wards.

Are psionics widespread, considered "just another form of magic", or restricted to certain races and individuals?

It's a form of magic that anyone can tap into, but only a select few races can teach it to a powerful level. So a dwarf may learn to read the thoughts of fellow dwarves, but a old gith can crush a building with his mind.