On Planar Metropoli and Their Varied Destinations

While Sigil might be the most obvious one D&D players from the era in which the TSR setting bonanza occurred there are other cities with multiple portals to multiple other realities. Union and the City of Brass come to mind from within D&D, but also Moorcock's Tanelorn. If we stick with WotC I'd add in Everway.

You could have portals/gates leading to different planets which gives you exposure to different cultures, species, levels of technology, gender differentiation, etc. Maybe places where physics and/or magic works a bit differently if you want to go the distance. What I think gets introduced with actual fantastical realms, "planes", is that archetypes, concepts, and abstractions are made physically real. There are Planes that function as Platonic Realms of Alignment (The Outer Planes) and/or Alchemical Realms of Elemental Substances (The Inner Planes). You could also have planes representing Technology, Time, Government, Knowledge, Math, etc.

And of course, you can visit the realms of your gods....which then requires one to answer the question of why the gods (for archfiends, or the Lovercraftian pantheon, etc) can't just enter the city and use it as a beachhead into other realities. Sigil of course had the Lady of Pain, and Tanelorn from what I recall had some ability to stay hidden. A particular deity, perhaps a mysterious and ancient one that younger gods fear, might own the city or even live within it.

Another alternative is that nothing prevents deities or their armies from entering a planar metropolis but such aggression would create a war in the heavens/hells/limbos//purgatories/etc that most of the time it's not worth it. This might also depend on how many portals a city has - Sigil is so coveted because it leads anywhere & everywhere, but others like Union are much more restricted in their offered destinations.

One of the other big decisions about planar metropolises is why don't people use them to go to the Heavens and get taken care of by the benevolent entities there? Some suggestions I've seen are celestials are too busy fighting the Manichean "Psychomachia" (War of Souls), that there is a natural order to be maintained between life & death and thus mortals immigrating to Heaven upsets it, Heaven's environment is actually inimical to moral life (rains of burning gold, rivers of light, etc), or that mortals literally pollute the parts of Heaven they enter and may even shift the real estate to Hell, the Abyss, and similar unpleasant locales. One might even put limitations on how mortals of the Prime Material Plane can utilize portals - there might be a time limit to how long one can stay outside their home plane, or perhaps portals only work for those who are astral projecting or vision questing.

The next question is how/if the peace is kept. You have Good & Evil entities in conflict, possibly elemental/fey conflicts like that between servants of Winter versus those of Summer or that between Ice & Fire. Order and Chaos may also be in conflict, especially if Order/Law is engaged in a Chaoskampf against those forces which would oppose civilization. Planar Metropolises may be controlled by a particular side in such conflicts, but the more conflict-neutral cities may have a peacekeeper like the Lady of Pain or just some fear-inducing taboo or conspiracy theory that keeps everyone in line. Or maybe besides the indestructible portals it's just a war zone with the gate ways continually changing hands as devils & demons rush into the mortal worlds and then, on the morrow, angels take those portals and head through in pursuit.

Part of the value of a planar metropolis will depend on the setting's cosmology. A setting with a land of the dead and land of the living may have numerous  openings between the two sides. Planar metropolises may not be seen as that important, though it would likely depend on the relationship between the two realities - if undead pour through from Death to Life or conquerors from Life try to invade the peace of the Dead such cities may be strategic choke points designed to keep anything which comes through a portal from getting outside the city walls. Cities that could lead to any plane may be both valuable but incredibly terrifying if the number of planes is immense to infinite - if not even the gods know all the realms out there what do you do with a city that might bring in new powerful invaders any day of the week? If the gods are the juniors to Titan/Tiamat/Jotun like precursor entities then a planar metropolis may be a linchpin to the ordered part of the cosmos drawn from the possibilities of a fecund Chaos. They might be placed on the edge of Lawful reality, beyond which is the Void of Chaos. Or perhaps they lay deep in the Cosmic Ocean, in the dark Waters of Chaos, and it is only in these areas of Deep Possibility where one can open up & maintain so many portals to so many places. Each city may then be an Artifact of Cosmic Xaos, ones which the forces of Law are eager to destroy if only they could find and enter them....