Cross-planar wars

Much like you Wicke, I liked the contrast between the planes but I couldn't find a lot of ways to bring those contrasts to full-out conflicts.

Let's see what we have:
*Mechanus vs Limbo
Modrons vs. slaad? The slaad seem viscious enough but not organized enough to start a war.
Would the modrons or inevitables finally decide that it was time to impose order on the slaad?
Would these battles happen on other planes instead of on Mechanus or Limbo (e.g. perhaps the slaad are seeking to make the Beastlands more chaotic and the mordrons are trying to prevent this - I find the idea of a modron in camoflauge as amusing; perhaps some of them will turn into clockwork-type animals)

*Arcadia vs Pandemonium
Again there doesn't seem to be enough people (and definitely not enough people sane enough) to form an army on Pandemonium.
Perhaps the Arcadians decide that they can "cure" Pandemonium? Perhaps in their misguided effects, the Arcadian "missionsaries" end up getting infected and bringing insanity back to Arcadia

*Mount Celestia vs The Abyss
Angels vs. demons - no-brainer

*Bytopia vs Carceri
I personally don't feel that the populations of either plane are very well defined in the cannonical works. I tweaked these far beyond the point where they would be recognizable to most

*Elysium vs The Grey Wastes
Since yugoloths are more of schemers than direct combatants (yes, I know they work as mercs, but I think their real power is in their mysterious plots); I could see this as a Cold War where the yugoloths and the guardians each send agents to subtly influence things to further their goals

*Beastlands vs Gehenna
I'm drawing a blank. Again, I tweaked this substantially

*Arborea vs Baator
I see the eladrin of Arborea sending in covert teams to sneak into Baator and disrupt their operations. Ideally, they would never be discovered.

*Ysgard vs Acheron
I changed things so that the infamous Norse giants lived in Acheron and that dimensional rifts would open periodically between the two planes so that the Norse warriors of Ysgard would charge into Acheron or the giants would try incursions into Ysgard. If one side was victorious, then that plane would gain a new floating island

I think it was in the Elemental Plane of Earth, actually, and it was a diamond mine. The modrons wanted it because they needed the diamonds to create more of their kind (which was an odd revelation, but whatever - it was good to see modrons in 3rd edition at all).

Heh. I was thinking that territory would never be a consideration here, but it's amusing to picture victorious Ysgardians towing an Acheronian cube across the Outlands sky to Muspelheim, or victorious Acheronians doing the same to a Ysgardian sky-island. (Or either one of them taking it back, of course.)

A Ysgardian sky-island might grow better provisions than provender cubes for your forces, and would certainly be a mark of distinction for someone able to organize the expedition to Ysgard to obtain one, fight off its defenders, and drag it back to Acheron.

...the Ysgardians would probably just do it for the kicks.

I realize I failed to include Mechanus/Limbo in my list, but now that I think about it, I could certainly see that as a common piece of planar warfare. On the slaadi's side, it would be unpredictable boilings-over during which a sizeable group of slaadi seem to just randomly take into their heads the notion of romping over to Mechanus and making as much of a mess as they can, while the modrons would organize parties that strike at specific objectives in Limbo to decrease the power of Chaos with calculated precision.

I also desire to drop into this conversation the notion of a single, giant braid war, consisting of an odd-length opposition repeated multiply around the Great Circle! With length 7 you'd get:

Elysium -> Gehenna -> Arborea -> Acheron -> Limbo -> Arcadia -> Abyss -> Bytopia -> Waste -> Beastlands -> Baator -> Ysgard -> Mechanus -> Pandemonium -> Celestia -> Carceri -> Elysium

Let's see... what justification can we weave up for this. Elysium's forces, in their most common offensive off-plane sorties, tend to attack yugoloth strongholds on Gehenna rather than the Waste itself. The 'loths, when they do bother to move openly off-plane, focused their efforts historically on Pelion and in recent millenia have been working on the Seelie courts, starting with creating the Unseelie Court. The elves of Arborea are their most common off-plane attackers, and they take the fight to the orc-kind of Acheron. Acheron's armies, mad in the way of iron discipline, claim they can pacify Limbo better than the modrons, and launch grim, expensive attacks on the home of Chaos to carve out power and magic. Limbo's inhabitants, in turn, are perhaps somewhat repelled by Mechanus, and when not beating off incursions from Acheron are more likely to glory in marring the beautiful rigidity of Arcadia. Arcadia defends itself stoutly, and focuses its efforts on fighting the evil and chaos of the Abyss (Pandemonium not being so big a threat).

Mt. Celestia is too well-defended for most demonic invasions, what with the Silver Sea, so Abyssal raids are conducted (usually through Astral pools) to plunder and loot the rich fields of relatively undefended Bytopia, despite Mt. Celestia's best assistance. Bytopians tend not to respond militaristically, but rather to work hard and wage cold war against the Waste, through productivity and charity, their businesses funding trade missions, relief organizations and propaganda teams that fight hopelessness, disease and death across the planes, even in the heart of them.

The Waste doesn't care. When it boils over and pops, its powers' tiny skirmish parties usually carry some plague or pollution into the pristine beauty of the Beastlands, taking delight in ravaging the colorful ecosystem there, rather than facing their Elysian opposites directly. While they sometimes respond, the powers of the Beastland are more concerned with doing their part in the wars against evil, and tend to send small hunting packs against the devils in Baator, who annoy them more than demons (or the less-blatant yugoloths).

Baator is far too powerful to care about most of those attacks, rarely losing anything significant to them. While it focuses the vast share of its attention on the Blood War, when it does strike against the good powers it tends to make the most efficient strike possible against the chaotic types, going for Ysgard just above the Good/Evil break.

I can't really think of a good reason for Ysgard to be attacking Mechanus with any regularity, unless it's perhaps Surtr and Thrym upholding the banner of chaos, or generally Vanir from Muspelheim rather than the better-known Aesir. In that case, I suppose it makes sense that the greatest proponents of order might anger them the most. They don't do a lot of damage, though, mostly petty strikes and thefts. Meanwhile, Mechanus sends modron mind-healers to Pandemonium with great frequency, in a long-standing project to understand the nature of madness and its healing. Naturally, the inhabitants object, so these parties are well-protected, and the whole affair can look a great deal like a war.

The powers of Pandemonium... why do they focus their spite on Celestia? Perhaps Auril and Talos assail their pantheonic brethren, goaded on to stupid bravado by Loki? Celestia simply defends itself and watches for escapes from Carceri, which are often sorties of evil headed directly for Elysium and the center of the goodness the prisoners were denied.

Hmm. That's a kind of weird setup, isn't it.

This may be obvious (and probably not what you wanted when suggesting this theme); but, another angle on this idea might be to focus on the different forces trying to influence things at a points of contention.
E.g.
A huge group of pilgrims are relocating (to the planes, to another Prime world, whatever) and they are trying to decide on the form of government to install.
I could see the Arcadians (and/or Harmonium) showing up to push some Utopian ideal. The Baatorians showing up to corrupt some select charismatic figures to convince them to set up a dictatorship (as a strong ruler will be "good for the masses"). And maybe the Arboreans show up to push a democratic or libertarian form of government.

A new magic item produces bursts of intense creativity in the people who touch it. Perhaps some Bytopians that encourage ingenuity show up to encourage productive, helpful invention while some Acherons show up to try to twist the inventions into engines of war (arguing that the people have to protect the item from those that would steal it).

For a potential Arcadia vs Pandemonium conflict
Above, I already mentioned my idea of Arcadia trying to "cure" Pandemonium and bringing back an infection of insanity to the orderly plane.
What if this was a plan set in motion by the gods of madness and bitterness that live in Pandemonium. They created a tempting target for "conversion", planted the seed of the idea in the mind of an Arcadian (think of an Arcadian who went mad in Pandemonium and was programmed like the Manchurian Candidate) and then the Pandemonites(?) waited for more victims to arrive.
The ultimate goal of the Panda-ites(?) is to cause one or more Arcadian cities to go so mad that they jump to the opposing plane.

""I realize I failed to include Mechanus/Limbo in my list, but now that I think about it, I could certainly see that as a common piece of planar warfare. On the slaadi's side, it would be unpredictable boilings-over during which a sizeable group of slaadi seem to just randomly take into their heads the notion of romping over to Mechanus and making as much of a mess as they can, while the modrons would organize parties that strike at specific objectives in Limbo to decrease the power of Chaos with calculated precision.""

I find it unlikely that the Slaadi would want to romp over there.
First of all, much of the plane is inorganic, and even if the Slaadi are capable of digesting metal and stone (which they might be, I don't know much about them), such things are surely far less tasty than flesh.
Second, the Slaadi are likely to find the very air about Mechanus to be uncomfortable.
Now, while the Slaadi do suffer from a certain degree of chaotic-stupid syndrome (mostly when it pertains to their stomachs), they still possess basic self-preservation instincts, and those instincts would likely drive them to avoid Mechanus and Acheron (there are more yummy things there, such as the Achaerai, but it's still a very inorganic plane outside of the battlefields).
The Modrons are only interested in the goings-on of their own plane for the most part, so they're unlikely to pick fights with the natives of Limbo.

""While it focuses the vast share of its attention on the Blood War, when it does strike against the good powers it tends to make the most efficient strike possible against the chaotic types, going for Ysgard just above the Good/Evil break.""

I'd think they'd prefer to strike Arborea to Ysgard. Arborea epotimizes the pure blend of both things the Baatezu hate-- good and chaos.

Regarding VikingLegion's idea, I could see this making a good adventure although it would probably curtail the jihad envisioned.
I could imagine that the pushy "drill sergeant" LG angel was killed by a fiend (probably a yugoloth or its agent) and then the eladrin that the angel was pushing is framed.
I could see the PCs investigating the mystery and calming rising tensions. The bad feelings and accusations involved would probably end the joint celestial effort even if the PCs are successful but at least the jihad would be stopped

Palomides: Ah heh. I see your point. Actual republic vs. tyranny, not Plato's version.

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The reason we don't see LG v CG very often is that people are, of course, uncomfortable with the idea of good people fighting one another. However, I could see this one:

War of Liberation

A joint force from Arborea and Ysgard is going to the aid of Arcadia -- they will be attempting to kick the Harmonium out, by force, and naturally the Harmonium will resist. Naturally, this will do even more damage to Arcadia, but the paladins of freedom claim that the net damage will be less than if the Harmonium are allowed to continue; they've just lost faith in the Harmonium's ability to change for the better. The Arcadians themselves don't really want it, but a few voices are enough to fire up the Ysgardian war machine. Rumors fly that there is baatific manipulation going on as well. Develop to taste for your campaign; the PCs' role is presumably to stop the coming conflict.

And here's one more general, neighbor-to-neighbor conflict.

Colonization

Now that it's known part of one plane can slip over to another, multiple groups of powers are considering colonies in neighboring planes as tactical moves. (In addition to the traditional efforts to collect pieces of the Outlands, by turning gate-towns.) The Upper Planes tend to avoid doing it to each other, and most of the Lower Planes have bigger fish to fry in the Blood War, but there are some hotspots of this new tactic.

Mechanus <-> Arcadia and Mechanus <-> Acheron: the modrons tend to stay at home, but formian colonies have been spotted on both sides of Mechanus, seeking to collect portions of those planes to expand the formian territory in Mechanus. Vice versa, desperate natives of Arcadia are trying to settle Nemausus and re-consecrate it to goodness (not so much stealing other chunks of Mechanus), while Archeronian armies are salivating at the thought of invading portions of Mechanus and occupying them long enough to rip out gears to serve as new Acheronian territory.

Baator -> Acheron and Baator -> Gehenna: the devils are willing to attempt this off-plane, but since they are mostly confined to their own plane the method tends to be sponsorship: they offer nation-level resources to territory-holding groups if they agree to swear fealty to Baator, begin expansion efforts, and turn conquered populations to devil-worship. Several small-time operators have accepted the deal, and are beginning to get noticed locally. No word as to whether their holdings are becoming noticeably Baatorian.

Abyss -> Carceri and Abyss <-> Pandemonium: various demon lords are attempting to create local concentrations of demonic influence on their neighboring planes, and due to the malleability of the chaotic side of the Ring some of these attempts have already succeeded! There are orbs in Carceri where descent from the first layer drops one into a long fall until the portal to the next layer is reached, because this orb's Cathrys is now floating in the void of a layer in the Abyss. Chunks of the Abyss and Pandemonium have been swapping so fast under demons and unseelie that there are regions where the two are intermingling almost indistinguishably. What there isn't is any evidence that these ventures are turning a net profit in soul-energy, belief, or other resources.

Ysgard -> Limbo: finally, there is a movement afoot in Ysgard to claim some of the primal energies for Good; one justification for this expansionism often advanced is that the loss from Arcadia must be balanced. Thus, hardy souls from Ysgard have been forging settlements off-plane in the soup of Limbo, establishing warriors' encampments and Norse churches, hoping to pull some of the soup of creation into the sky of Muspelheim. Only a few are voicing much worry that they might succeed and bite off more than they expected; even the slaad are putting up surprisingly little fight. Perhaps they know something the Ysgardians don't.

I have been trying to drum up a few ideas for a Gehenna /Beastlands conflict but I’m still fishing. I think the main problem (for me at least) is that Gehenna is so ill-defined in the standard cosmology. Sure it’s got volcanoes and many of us have tied in themes of struggle/exploitation/intimidation but how does that make it unique from the fiery regions of Baator and their tyranny?
I think Gehenna may need a visit from the Planar Renovation Project.

Personally, I tried (not entirely successfully) to focus on themes of brutality for survival in Gehenna. Not the will to dominate another (as I feel that is Baator’s job), but rather a rationalization for any evil as a means for one’s own objectives. This would start at the basic “survival of the fittest” level where one justifies killing another for one’s own survival (which it shares with the Beastlands) and moves up the scale to the Mafia mantra of “It’s nothing personal, it’s just business” attitude of wiping out rivals for trivial objectives like more money.
I liked the Mafia connection as this also tied it to the theme of slight lawfulness (in this case, lawfulness meaning being loyal to the crime syndicate group).

By contrast, I added more focus on the Beastlands as the realm of the “noble savage”. Whether such a being ever existed, the concept definitely did. The noble savage threw off the trappings of society (which they felt corrupted people) and had a natural report with the natural and spiritual world.
(In my reworked model, I tied the Indeps and their resistance to embracing the factions to this reworked plane).

I mention all this as I think the contrast of “noble savage” versus “mafia underling” (who allows evil because of his slavish association with a social institution, i.e. his gang or mafia family) seems like riper ground for a conflict than nature vs. bullies or nature vs. pollution (which I don’t think is currently associated with Gehenna).
To me, they both start at the same "survival of the fittest" point where an evil is needed to survive, but then split. The Beastlands don't allow any greater evils as they realize that other things they want (mansions, luxury, etc.) just aren't that important. Gehenna, by contrast, figure that the evil they commit isn't that important so long as they get the things they want.

Having said all this, I still can’t think of a good reason for direct conflict. I feel like I'm close to something, but I'm still not there yet.
I could see both groups trying to sway a community (e.g. a logging town turns into a conflict between the druids/elves and the greedy logger-barons) but I don’t see a good reason for the bullies of Gehenna to try to grab/convert the natives or territory of the Beastlands. And I can’t see why the free-wheeling natives of the Beastlands would bother with Gehenna.

Any ideas?

If you really want to go this way, I can't remember the name, but I seem to recall there being a construct race created by the Archons that ended up warring with them (or maybe just fighting them) because they ended up becoming CG. There was a peace between them brokered by the Guardinals, I believe.

I wish I could remember what that race was called, but I think they were in Planescape MC2.

Edit: Whoops, I was a little off. Found them: the Quesar, from the Planes of Conflict MC. They were an _NG_ race created by the _aasimon_, not a CG race created by the archons. The rest was basically right, though; they didn't want to be servants of the aasimon, they rebelled, there was a war, then some gods in Elysium brokered a peace between them, and the quesar stayed behind in the marshes of Belierin where they'd been created in the first place.

Ah, and therein lies the dilemma: This group ends up finding some (dark) knowledge/item/etc. known only to the top few paragon celestials (and maybe the powers), who don't want the information getting out to any other beings in the Multiverse.
The fear being of course, (primarily) that should the "white hat hacker" group be captured and interrogated by say-- fiends or some such, that they might leak out the info.

I was thinking more along the line of NPCs doing that, and the PCs have to... mediate or something or... somesuch.
Maybe I wasn't clear enough, but the secret that this group uncovers is supposed to be some deep-dark secret/closet skeleton/intrigue/conspiracy about a power or quasi-power with multiverse-shattering implications.
It could be as benign as the fact that some horrible ultra-evil major artifact is being 'safeguarded' on one of that plane's most sacred areas-- an artifact that was say, supposedly destroyed.
This could be particularly damning if it occurred on Celestia, because that would mean that the Tome Archons at the very least lied to the public (a very un-lawful good thing to do!)