Using ideas from Beyond Countless Doors & Tome of Battle?

The only tricky parts are the places that would have to be in the Outer Planes, particularly Curnorost (the realm of dead aasimon) and the Ten Courts of Hell. Curnorost, a realm of misery and despair only accessible from the celestial planes, would probably be part of the Gray Waste (or possibly Pelion), while the Ten Courts would go in various parts of Baator.

I would tie the Ten Courts of Hell with the Court of Judgement in the Outlands. They would have to be the main way in, and the main source of the Ten Courts' prisoners. On the other hand, I would open up the various other courts a bit, so they could interact more with the rest of Baator's population. That's the main benefit of having seventeen big outer planes with lots of little realms in them, instead of having a seperate plane for each god, planar species, or pantheon. - you get more interesting interactions.

The First Court probably actually exists within the Outlands' Court of Judgement - it's just the part where sinners are sent before being shipped off plane.

The Second Court, the court of slave breakers, exists somewhere on the River Styx, probably in Avernus or on a cube in Avalas. The fact that its ruler is half slaad would make Avalas more likely - Avernus is a more lawful plane than Baator, but it's also more accepting of foreign mercenaries.

The Third Court would be in Avernus, probably also on the Styx, where the baatezu come to buy agricultural products from it. It could also be in Stygia, since it fits in some ways with Set's realm.

The Fourth Court, known for its academies of sorcery, I'd put in the City of Dis so that many fiendish creatures could benefit from it. The Fourth Court is also the home of many rakshasas, so there is probably a lot of traffic from the rakshasa cubes of Acheron.

The fifth through ninth courts are noted as constantly plotting to undermine one another and depose their respective rulers and steal their prisoners. As such, their location is somewhat in flux: they could slide to other layers or planes depending on whose plans are successful.

A place of fire and torture, the Fifth Court is probably in Phlegethos.

The Sixth Court, filled with serpents and yuan-ti, might actually be in the Abyss, probably in Merrshaulk's layer of Smaragd. It could be in Minauros, but its weather wouldn't be as sunny and pleasant as it's made out to be. Conceivably, it shifts between the two over the eons.

The Seventh Court I would put in Baator's seventh layer, which also has a fallen celestial ruler obsessed with perfection. In fact, Triel and the Bronze Bell King might even be one and the same. If they're not the same, Triel would still appreciate having the Bronze Bell King within his domain. One adventure hook that occured to me was having Autochon the Bellringer hire the PCs to venture to the Seventh Court of Hell to find a way to end his curse, since the Bronze Bell King is constantly experimenting with a magical bell, attempting to make perfect tones. If anyone can heal the curse of Sigil's Demon of the Bells, it's him. The tssng might also be interested in the layer, volunteering to help the Bronze Bell King with his project or being kidnapped by him for that reason.

The Eighth Court, a realm of utter cold, closely resembles Baator's eighth layer of Caina. That's where it belongs, and the rivalry between the Bronze Bell King and the Snow King might closely resemble the rivalry between Triel and Mephistopheles.

The Ninth Court, a realm of utter darkness, would fit in one of Nessus' trenches.

The Tenth Court is a mysterious place where no one has entered and returned, so it isn't really important where it is - probably in a deeper pit of Nessus, though, or in a completely unexpected place like Mechanus, near Shang-Ti's realm. The Eleventh Court is just a rumor, but it's likely in one of the upper planes, particularly Mount Celestia, probably near or in Kuan-Yin's realm.

The Crystal Roads of Deleur are also somewhat tricky. They're either part of the Plane of Mineral, or a demiplane. I'd put them in one of Mineral's border regions.

Some creatures that should be added to the Lizard Kingdoms' encounter list: yuan-ti, sarrukh, abeilan, vermin lord, saurial.

The planar trade city of Carrigmoor I've been thinking about a lot. I've decided to blame the plague that largely destroyed it on the mercane, so they could bring its most valuable citizens to their newly created city of Union. The plague could also be related to the one the Lady of Pain used to destroy much of the Free League, or to the Iron Shadow from Tales From the Infinite Staircase. I've tied Carrigmoor's creation to Sigil's Great Upheaval - it's where most of the guilds and exiled factions went after Sigil's factions were reduced to fifteen.

In the beginning there was the Rose; her buds and thorns made up the whole of the multiverse. Her blossoms were the blossomings of worlds; her roots and stems transversed all boundaries.

From the Rose was born Aoskar, the Opener of Doors. The Rose was Aoskar's teacher, parent, and friend.

"Rose," said Aoskar sadly. "The time has come that I must leave. I must seek the new places you have told me about, that the inhabitants there may find their way to you."

"Yes," said the Rose. "It is time. Good journey, beloved Aoskar." Before he left, though, the Rose gave him a gift: a tiny, flowering vine that she named Folly.

During Aoskar's long wanderings, before he found his throne in Sigil, he sought an oracle to show him the path he must take. He consulted the Norns, and the Gray Sisters, and the Sibyls of Elysia, but none would say what the destiny of the god might be. "Your future is forbidden," hissed Skuld. "It is guarded by Pain."

Shekinester, triple-goddess and World Serpent, divined something of Aoskar's fate, and harried him and his servants with her ha-naga proxies. "Is it war she wants?" mused Aoskar. "I can give it to her."

"I would be careful," cautioned Folly. "She is strong, and old. Better to attempt to bargain."

Aoskar followed Folly's advice. Calling in a favor from a being called the Wandering Architect, he had a sumptuous meeting-hall built. In it, he opened a number of doors that would allow him access to all the powers that he had within the Rose itself. Sending a two-headed hound to Shekinester's Court of Light, he invited the Three-Faced Queen to complete the hall with her own powers, and meet him on what would be neutral ground.

Intrigued, Shekinester did so. The meeting-hall became a place of seemingly endless maze-paths reminiscent of her own Loom of the Weaver. The goddess herself appeared within its heart. A young girl with reptilian features floated to the entrance and bade Aoskar walk in.

Venturing through the maze, half familiar and half new and strange, Aoskar felt his own doors change and twist. He felt as if the Rose was calling him somewhere, or warning him, but her words seemed jumbled, reduced to incomprehensible symbols that floated through the streets of an alien city, building and rebuilding its walls and doors. And in the center of it all, a circle of blades. Something about them filled him with terror.

"You should go back," said Folly. "The Rose wants you out of this terrible place."

"I can't," said Aoskar. "This is my destiny. If I leave now, I'll never understand it."

And so he passed the tests of the Empowerer, and passed into the presence of Shekinester herself.

"You have great courage," whispered the goddess, her human face shifting from young to old and in-between. "You will need it, where you are going."

"Is this why your minions have been harrying me?" Aoskar asked. "So you could warn me?"

"Not warn, merely test. I wondered what kind of god would go to dwell in the City of Doors. None have done so since the Phoenix's last death."

"Were your questions answered?"

"Almost," said Shekinester. "Before we are finished, you must face the Arching Flame."

As if struck by the same impulse, Aoskar and the serpent-goddess embraced, his limbs circling her coils, her coils circling his limbs. Aoskar's two faces looked into Shekinester's three faces: maiden, mother, crone. Then she was a pillar of light, and he and she were one.

After an infinity of union, the light faded, and Shekinester was once again a three-faced naga. Aoskar was once again a dual-faced man.

"I feel cleansed, healed," said Aoskar. "I wish we could merge forever."

The Empowerer laughed; the Preserver smiled gently; the Weaver sneered. "We each have our destinies to fulfill," they said. "But this place we have made will remain. As will one other."

The girl who had met Aoskar at the first door reappeared. "Farewell, Father," she told him.

"A child has been birthed from our unity," Shekinester explained. "Dislocated slightly in the warp and weft of history. Meet Aoska-Shek, for the second time."

Aoskar awkwardly patted his daughter's head. Something about her made him uncomfortable; probably it was the way her feet never touched the ground.

He turned to look at Shekinester, but she was already gone. Shrugging, he left the Nexus they had made and went to meet his destiny.

I thought the reth dekala were the most interesting addition. Being from Acheron and made of smoke and iron and flame, they reminded me of Vorkehan, the Mercykiller city.

My idea is that before the Great Upheaval, the Sodkillers were a faction whose core belief (perhaps inspired by the kolyaruts) is the importance of contracts. They also had a job - mercenary work - just as the Godsmen make tools and the Takers collect taxes, but that's not their philosophy. The Sodkillers, then, believed that the multiverse itself was a client and it was important that both sides fulfilled their contract, just as it was important for mercenaries to fulfill the contracts their clients gave them. Just as a mercenary must be paid, so must all of us if we do what we agreed to do. The dissent might be along the lines of: what exactly have we agreed to? When did we agree? Is our payment a lifetime, or is it the afterlife, or both? Is the afterlife a different job than life, or is it what we get in exchange for living? Is there a true afterlife, or is our wage blessed oblivion as the Dustmen claim? If death is our reward for success, what is our punishment for failure, especially if you believe that Acheron is the primary exemplar of your philosophy (preventing the Upper Planes from being seen as a reward)?

It's not hard to see how such a faction might find common ground with the Sons of Mercy, resulting in a faction that believes in justice first and foremost - the proper reward for fulfilling a contract, and the penalty for failure, is obviously part of that, but the Sons of Mercy bring in the new idea that there are also laws you haven't agreed to that are equally important, as well as the possibility that the less severe punishment might be given when laws are broken - a strange concept known as mercy.

This, by the way, is not necessarily the philosophy of the modern Sodkillers, who are really just the stern no-mercy fraction of the Mercykillers and not a true rebirth of the ancient Sodkiller faction. Though they're mercenaries again instead of running the Prison, they still believe in justice first and foremost; contracts are only one aspect of their philosophy, the one they're able to concentrate on the most in the new, less lawful post-Faction War Sigil.

Anyway, the story behind the reth dekala is that they were human mercenaries who entered into a contract with a baatezu noble whose stronghold was in Acheron (an exile?). When the baatezu started demanding they kill their own families, they refused, and so were cursed to exist in constant flame and agony. Though the baatezu is now long dead, they must hunt down and destroy their now-scattered descendents if they are ever to be free.

Okay, so how are the Sodkillers connected?

- Perhaps the reth dekala were Sodkillers, originally, and the Sodkillers of Vorkehan are those mercenaries who didn't turn against their patron. Perhaps Vorkehan was created by the baatezu noble, then - even its stronghold. The Sodkillers/Mercykillers might consider the reth dekala to be foul lawbreakers who deserve any agony they might inflict, while the reth dekala might revere the Sodkillers for having the strength and foresight to do what they could not, resulting in a weird sado-masochistic "Please punish me/I must punish you" relationship.

- The reth dekala might have approached the Sodkillers for help in fulfilling their contracts, and helped to build their city in exchange for helping to hunt down their descendents. When the Sodkiller faction stopped existing, they were technically free of this requirement, but now that they're back the reth dekala are insisting they continue their ancient task.

- Those reth dekala who decide to endure the pain rather than killing their descendents might be hunted down by enraged Sodkiller/Mercykillers, as well as kolyarut inevitables.

- They start a faction dedicated to the idea that planes are the ultimate form of life. They seek to evolve lesser beings into planes, and awaken the intellects of already existing planes.

- They gang up on Bytopia and steal its lunch money.

- They all develop multiple personality disorder.

- They throw a party and deliberately snub Neth, the Demiplane That Lives.

- They merge to become a mad Limbo-type plane.

- They secede from the multiverse, creating their own cosmology.

- They converse quietly until the conjunction is over.

- They studiously ignore one another.

Another possible tie-in between Planescape and Beyond Countless Doorways is to say that the demiplane of Faraenyl was created with the aid of Logicus from the adventure Doors to the Unknown. The two make a good pair - in Logicus technology works but magic does not (except for within the World Mind that holds the planet together); in Faraenyl magic works but technology does not (except for within the Grand Cycle that holds the demiplane together). Perhaps the natives of Faraenyl originated in Logicus, but fled to an artificial demiplane as the world was dying, needing technology as well as magic to create a demiplane large enough to suit their needs.