Oppositional Symmetry

Actually, the idea that Fire=Hell is inherited from the western Christian bias. As was previously mentioned, Baator itself only has a small majority of fiery planes. Looking at the layers:

Avernus: Fireballs bursting over a rocky wasteland

Dis:  Iron City

Minauros: Swamp

Phlegethon: Fiery hell in the Christian tradition

Stygia: Icy Ocean

Malbolge (pre-Glasya): Endless rolling boulders (post-Glasya) oddly organic land of consumption (hard to describe)

Maladomini: ruins throughout, some lava flows

Caina: Frozen waste

Nessus:  Yawning chasms

While this list is simplified, only Avernus and Phlegethon are fiery, balanced by Cania and Stygia. Maladomini and (pre-Glasya) Malbolge have some firey rivers, but these aren't a significant element of the layer. OTOH, since baatezu are immune to fire, it would make some sense that the layers have some fiery elements.

As far as the larger query about opposing parts of PS: The elemental opposition concept is inherited from ancient Greek philosphy. Opposition on the outer planes - even Christianity has heaven and hell. Most mythologies have a Paradise and a land of evil/dead/whatever - PS simply blends them all together in a convenient package.

To address the Baator in particular, it's at least loosely based on Dante's Inferno.  Most of the layer names and features are the same or similar to what Dante writes.

As someone above mentioned, it sounds like the 4e cosmology would be right up your alley.  The 4e Manual of the Planes is out, so you can take a look at the new "official" cosmology at your leisure

This does bring up a good segue for a point I've been discussing with a friend of mine, though it may belong in another thread entirely.

Simply put, a several of the planes are largely boring or easily overlooked compared to others.  Some of them feel better-developed or at least much better-suited for adventuring. In other cases, it's hard to create a planar adventure that actually feels planar instead of simply being easily-transposed onto a Prime (something I have personally dealt with). Bytopia, for example, is a neat plane full of hard-working folks, but it's not terribly inspiring on its own, nor is it really all that different from a Prime -- except that you can look up and see the plane's other half.  Arcadia is similar, even with the Harmonium presence.  Rows of neatly-organized orchards is not a terribly exciting prospect. 

Many of the Outer Planes, especially the Upper Planes, suffer from this.  Most of them can be summed up in a single sentence which is good in some circles, but makes for rather bland stretches of infinity.  Many planes (again, especially the Upper Planes and Inner Planes) lack a clear and defining conflict that's noticeable at a glance.  This, I suspect makes them seem superfluous in the eyes of many Planescape critics. 

I'm going to pick on poor Bytopia again.  There's some conflict there, but it mostly portrays itself as a happy little place of a bunch of people working hard toward essentially the same goal -- hard work for hard work's sake.  While it's a workable concept, it's not terribly interesting for an adventure in and of itself.  Creating an adventure would require a seriously disruptive outside influence (fiendish invasion, et al.).  All things considered, I don't see the Great Ring as written suffering tremendously if several planes were cut off completely. 

Some planes have conflicts that are downplayed or glossed over.  Take the war between the facets and the Plane of Water.  To me, that has potential to be an excellent major conflict running throughout the Inner Planes -- and it's something I'm playing up in my game here.  But the facets are so obscure that even many loyal planewalkers barely know what they are. 

Again, this is more of an issue for the Upper Planes and Inner Planes than the Lower Planes.  Most of the Lower Planes are full to overflowing with all sorts of conflicts.  And it's apparent which planes are more popular and more usable because they're seen more.  Baator, the Abyss, Celestia -- these places are not simply popular because they're the homes of devils, demons, and angels.  They're popular because they're incredibly easy to use in comparison.  The current overarching nature of many Upper Planes feels innately geared against any sort of conflict, and what conflict exists can often be replicated on a Prime world.  As a result, it doesn't feel very special going to these planes.  The Inner Planes are static and unchanging, and what potential major conflicts exist are barely explored.  Take the facets vs. Water example.  If the marids decided they've had enough of salt creatures trying to destroy their plane, it'd create a great war that'd provide an easy and very usable reason to go to Water or Salt.

This doesn't apply to all the planes, of course.  The most notable exception that comes to mind for me is Elysium.  It's supposed to be a plane of ultimate contentment and relaxation, so a lack of conflict makes it work.  It's still not especially exciting for running adventures, but conceptually it serves its purpose perfectly.

I'm sure many folks disagree, or they see the intra-planar conflicts as more apparent and less subtle than I do.  But even having read the planar supplements several times (Planes of ____, Inner Planes, etc.), I feel many planes need something added to their overarching theme.  I think many planes appear unnecessary not because of oppositional symmetry but because there's no internal opposition.  Given a chance and contributions from others, I'd be willing to sit down and conference on how to rewrite "boring" planes to feel more "interesting." (I use quotations because your mileage may vary).  My friend and I already have some ideas.  But while many planes have neat individual sites, I really feel they lack something to bring the whole plane together as an appealing whole.

Like I said, if folks surprise me and actually agree with some of my points, I'd be willing to commit to a long-term group effort to redevelop the Multiverse enough to make it wholly appealing while still keeping true to "the way it always has been."

The issue with D&D Primes sharing the cosmology is actually addressed in PS.  The campaign said the inhabitants of those Primes may or may not actually understand how the Multiverse really worked.  Judging from the tone set in the PS books, Toril and Oerth were closest to the truth, while Krynn was among the most Clueless.  For example, Planewalker's Handbook says that Krynnish Primes refer to all the Outer Planes as "the Abyss."  Athas, IIRC, is situated closer to the Inner Planes than the Outer and is said to have "odd" names for some of the Inner Planes.  Essentially, though, the gist is that everything expressed in Planescape "overrides" all the Prime world cosmologies.  Then, if a Prime world defines their cosmology differently from Planescape, that Prime world is wrong and "Clueless" to the real dark of things -- including Earth.  All their gods dwelled on Planescape's planes, not

The 4e cosmology is not terribly difficult to reconcile with the Great Wheel.  The Astral Sea is analogous enough to the Astral Plane to fit fine.  All the Outer Planes were located "within" the Astral Plane anyway, and the "astral dominions" of 4e serve a close enough purpose to the Outer Planes that the change there is pretty academic.  The Elemental Chaos is basically "what if the Inner Planes didn't have boundaries?"

My intent would be to work with Planescape's existing structure and add the details and conflicts to the planes to make them more immediately interesting.  Not so much to reconcile Planescape with other cosmologies but to enhance and address what I perceive are some of the biggest issues in the Multiverse.

Arcadia is law tempered by goodness, so it's not necessarily the Harmonium philosophy of "good for everyone."  And actually, I personally think Arcadia's theme should be more benevolent than "forced" anything despite the Harmonium presence because of its position as an Upper Plane.  If anything, "forced" government is more the province of Acheron or Baator.

When my friend and I talked about this, we envisioned Arcadia as sort of "anti-Acheron," a land of magnificent kingdoms, each running under their own benevolent codes of law.  The kingdoms fought and wrangled politically, but the fighting was always noble and dignified and followed the rules of war.  Political wrangling happened, but it was neither vicious nor brutal and always stayed close to the law.  Chivalry was held in extremely high esteem and fighting for one's nation, one's king, one's people or even the plane itself was considered the greatest deed achievable on the battlefield.  It became a place where philosophers from all nations debated what laws were best for their societies and for other societies, too.  It was society first, certainly, but it was different views of what "society first" meant, and all with goodness at heart.  Treaties are made, remade, broken, and established once more as the kingdoms and nations constantly shift.  People that flaunt the law are expected to face punishment -- not out of spite but because these realms are all founded on the basis of law and they believe that benevolent order -- in whatever flavor -- is the most "good" thing that a person can have.

Of course, "the most good for society" is hotly debated today in the real world, so you can see where a good source of conflict comes from.  Some leaders might be more strict than others and while none are cruel (save that for Acheron and Baator), very few nations agree 100% on what it means to govern in a way that is in the people's best interests.  One nation might be fascist, another might be communist.  One kingdom could be socialist while another is capitalist.  One nation could have a hereditary petitioner monarchy that goes back generations while another nation is ruled by an elected Senate.  It allows room for all flavors and national styles and each one should have representation.  It allows both hot and cold wars.  It also opens up a bit more for planar adventures within Arcadia as PCs could get caught up in a war between two Arcadian realms.

Despite the differences, though, it's important to remember that the Arcadians are good people.  Rulers are generally LG, and they are LN at worst.  The plane's nature makes it impossible or extremely difficult for an evil creature to rise to prominent power similar to how Elysium naturally inhibits evil creatures trying to travel through it.  Perhaps Arcadian petitioners have a way of detecting falsehoods and deception that evil creatures perpetrate.  Regardless, the leaders are good folk and when evil strikes, the kingdoms are more than capable of uniting and standing as a solid front against the fiendish incursions.  This opens up ideas, too, for a war between Arcadia and Acheron.  Nothing so prolific as the Blood War, but you have two heavily militaristic planes standing a (figurative) hop, skip, and jump from each other.  On the occasions that an army on either plane can break off from its internal struggles long enough to look outward, I think they'd see each other as the next target.  Arcadians would see Acheronians as cruel tyrants who use the law to oppress their people instead of protect them (and they'd be mostly right), while Acheronians see Arcadians as weak, simpering pansies too hindered by their own laws to become the fighting force they could be.

In any case, while that idea introduces conflict, to a plane I feel is otherwise one of the duller ones, I think it may fall a bit short of being "planar."  Warring kingdoms is nothing new and is quite a common feature on Primes.  The fact that the Arcadian kingdoms are universally good and have their own interests as well as their opponents' interests in mind (no burning/pillaging/sacking occurs on Arcadia, for example) may not be enough to define it as a "planar" world.  Establishing conflict with Acheron helps (reinforces benevolent law and warfare vs. tyrannical law and warfare), but would those two things really set this idea of Arcadia apart from a "vanilla medieval Prime?"

Anyway, that's my very long two coppers on how I'd redo Arcadia

I still think that emigration/inmigration should be a conflict source in the "life is good" planes, hehe. 

I like how you've drawed Arcadia, and the Arcadia/Acheron conflict. But as I see it, it's more LN than LG. More a firm hand goverment to keep everybody lined up in the perfection. 

Well, this one is for Tequila. Have you read "The Death Gate Cycle"? It's some kind of concept of planes aligned to elements that look like worlds. "Earth" it's some kind of subterranean world with lava seas, "Fire" is a world like Arborea. Maybe you can make a coctail with those ideas and the remaked multiverse of the 4th ed. In those books, there are conflicts in every plane. In the "Air" for example, world is an infinite sky with floating islands, and the dwarfs living in the bottom think that people living in the supperior islands are godlike with their flying machines and so on.

I'm going to shanghai this part of the discussion into my other thread so as not to derail Tequila's thread more than I already have