Ok, so given the history of my setting (gods were highly interventionist for ten thousand years and it's been only a year since they lost their power) I think I'm probably best served by the basic concept of Sigil, the idea of a planar metropolis created to get away from the gods. That's going to give the planar metropolis the longest deity-independent history in world. I suppose I could also go with something like a city that was the home of a god of commerce, which allows for some interesting things like a banking system that had been backed by the god now in chaos, and a bunch of shit the god had set up or was controlling that now people know fuck all about.
Given that they're supposed to be going after the Ur Priest, said antagonist would be more likely to hide out in a "No Gods Allowed" place than the city of a god.
I'm tempted to use Xaxox, the lost fortress in the Far Realms, and just turn it into a planar metropolis, but that's only because of the NPC who lives there, and I can fucking put him anywhere. So no reason to invoke that shitplane.
The other couple ideas I've got right now are promoting Balefire to a planar metropolis because "the city of lanterns in the plane of Shadow" sounds really fucking awesome. Bonus points for not inherently dicking the Paladin over; a Fae Market, because, well, faerie market; and bringing in something like Ravnica as it's own demiplane. I'm not too sure about the Ravnica idea since in its source material it's just a plane that is a big city, not really a planar metropolis with the wish economy and such. But, if nothing else, it gives me some inspiration even if I don't name check it.
So Sigil has the Maltheist Utopia thing. It was created as a haven from the gods. Off the top of my head, I guess either someone who actively disliked the gods taking care of everything went off and founded it, or someone who was an enemy of the gods did so. Actually, no, fuck that. Sigil was created by the Vashar, because the Vashar are conceptually awesome (to me). Then I can pull a bunch of stuff from Hyzmarca's rewrite of them. That gives the Ur Priest antagonist a strong tie to the place. I like that.
I'm tossing the idea of the Town of the God of Commerce, because I'm not sure where to go with that.
I'm going to briefly entertain the idea of Xaxox because it's a place created by mistake. The manual of the planes says that it is a "relic from the Material Plane" and that it's just a big fortress
lashed to a giant desiccated tree. Basically a bunch of wizard found the Far Realms and said "LETS GO POKE IT WITH A STICK!" and promptly got lost there and went insane. The leader of the expedition was an elven wizard named Daruth Winterwood whose "brain is full of spiders. Literally." Which is just such a great character idea I think. It gives me the idea that his brain was actually replaced by a colony of spiders, with different memories held by different spiders, and sometimes they wander off. It makes me think of a Neil Gaiman thing,
The Man Who Forgot Ray Bradbury, so he'll be talking and then he'll just forget a word, because the spider that holds that thought wandered off for a bit. "Oh! The soul market! Yes, it's just down the- down... down the... river. Path? Vein? Wide thing... long.... cobbled... we're standing on it...." "Street?" "Street! Yes! Sorry, that spider wandered off. It's just down the... thing, hang a left after the third... campfire. Brazier. light.... fuck. Pole with a candle. Go for a kiloliter. Shit. Kilo... distance. And it'll be the place with a bunch of, er, shiny... stones." "Gems." "YES! Those. There you go!" So, Xaxox wasn't a planned city by a long shot. The expedition got lost, and they're trying to get the fortress back, but they totally figured out how to open portals, so people have come in, and it's become a known outpost in the Far Realms where things are sort of stable, and maybe the wizards even eventually thought to set up a protection against the Maddening nature of the plane, they were too far gone, but it's damned hard to conduct commerce with someone when they randomly start vividly hallucinating body horror. It's a total mage utopia, and has the highest level casters in the setting thanks to it's flowing time trait where no time passes on the Material no matter how much time you spend there (I'm purposefully not thinking about how you... handle the FR side progression of time...). People have caught on, to some extent about this, so people will go to study books, rest up, and all that, but it's mostly planar creatures, since mortals don't get much higher than 8th level on the Material. Add to that the fact that Far Realms entities will occasionally go Godzilla on the place and the wizards have been severely hampered in Project: Get Back to the Material. And then of course, the creatures who know about the FR's timeless nature don't want to lose the place they go to get a full rest up inbetween combat rounds, so there are tons of people sabotaging the efforts of the mad wizards who want to get back to the Material.
Balefire is described in Dragon magazine as being founded by refugees from a war-destroyed Underdark city. A gnome wizard owned a building there and started selling lanterns, which became popular and eventually so numerous they awakened a nightcrawler which came and fucked the city up. A powerful mage fought it off and declared themselves the new ruler. They have a signature mount called beacon moths, and with the lanterns I have this image of an asian aesthetic Bas-Lag in perpetual night that is really cool. Basically, it's another accidental city, but instead of Wizard Town it's Night Town, and so a bunch of underdark creatures live here. However, given the inherent low level of characters in the setting, I think I want it to have been founded by a fugitive godling of night or something. It's the Night-dweller's utopia, so there's probably a lot of orcs and kobolds and vampires and stuff that live here, but also svirfneblin and probably anthropomorphic bats and stuff. Actually, here we go. The hobgoblin deity of night and shadow, Ninshigami, was injured in a fight with another god, and his followers picked him up and carried him off to the plane of shadow to recover. Ninshigami decided that the hobgoblin pantheon could go fuck itself and created the town of Balefire in the plane of shadow. It grew as more night-creatures flocked to it as a haven of nocturnal activity that didn't require dealing with mind flayers and aboleths and shit. So it started as "Hobgoblin Tokyo in Perpetual Night," but a bunch of other cultures have gone in and now it's looking more and more like some cyberpunk version of Tokyo with haphazard buildings that clash with the background and were built by and for races that have different needs and values from hobgoblins.
The Fae Market is the "Alien Utopia" where Unseelie can bring their gutted and skinned halfling and dwarf quarry to market and sell hound archon pelts, and the Seelie can do their mortal-diddling without pesky iron-wielding dads getting in their business, and trap mortals by giving them food and shit. It's the place where even the "good" guys are probably dick-bags. But they've been at it so long that there's actually starting to be a mortal immigration problem. I could see the Fae having a "Stay out of our business and we'll stay out of yours" deal with the gods, probably with their own near-god rulers. The other problem for the Fae Market is that iron fucks them up. They probably attempt to confiscate all iron from visitors, and having iron inside the market is a definite status symbol, but Summon Weapon isn't a high level spell, so it can be difficult to protect against. And with the aforementioned mortal immigration problem, this is getting to be an even bigger problem as mortals are expendable, and few if any fae complain when one is killed, but there's no institutionalized gatekeeping preventing them from gaining instruction as bards in the Market.