Both of the examples below are interesting options for creating a unique city or world for fantasy adventures. A lot of good stories take an odd concept or what-if and just apply everyday logic to it to see what grows and takes shape.

Bones of the Dead

And if some legendary heroes downed a Tarrasque [stats], then this could be the template for the city that emerges from around the bones and keeps it contained. This thread about a city in the D&D universe which is built around the Tarrasque provides great inspiration for a city built around a single thing.

Tippyverse

Then there is the Tippyverse, which is described in this very long thread about all the major cities on the world connected by teleport circles and all production is handles by magical traps. I guess they trigger a magical effect, like fabricate/create food & water/etc. It’s a world where the wizards rule all and are heavily involved with the control of the cities

Alternative Places

But even the two great examples above are still bound by the classic ideas of what a city is.

Does a city need to fill a large area of space? Can a city use magic to exist on the surface of a coin or other small object? Or drawing from the stories of the faeries, can it fill the dreams of others?

Does a city need to be bound to the material plane or can it occupy another reality? This could give rise to various planar adventures as the player reconnect cities that are in different realms with differing realities.

Do the city need to be alive? This can lead to a city of the dead (or undead), a ruined city from ancient times, or an empty automated city waiting for it people to return {Miranda from the movie Serenity}

Earthdawn, Shadowrun, Torg are all good examples of starting with a simple idea and seeing where it leads. All the best stories are What-ifs.