I've recently been reading some of the Ethshar books by Lawrence Watt-Evans (all of them are stand-alone, fun reads if you haven't read them) and one of them mentions "The Edge of the World". I was kind of intrigued by that idea as well as the region called "The Small Kingdoms" where there are a couple of hundred tiny kingdoms crammed into a relatively small geographic area. Each kingdom has its own laws and customs and, in some cases, language.
There is another book in my collection, this one by Larry Niven as part of his Known Space series, called A Gift From Earth about a planet colonized by humans. The trick with this place is that the entire planet was uninhabitable because of the very dense atmosphere, except for one VERY high plateau that sticks up high enough that the air there is breathable. It provides a habitable area roughly the size of California. They have a litteral edge to their world.
These ideas collided in my brain in a sort of "got chocolate in my peanut butter" kind of way that has me envisioning the following:
What if my world is like that where the habitable portion is up high enough that the air isn't poisonous. But instead of one big plateau, it is a series of smaller ones. Perhaps dozens or hundreds of smaller ones. Some of them comprise their own kingdoms and each has a different name. Some of them are big enough to be home to multiple kingdoms that may or may not be at peace with one another.
Not all are at the same height. Some are down in "The Mist" where the air is hot and thick. And they are not uninhabited because that is where "The Mist Demons" live. These are, literally, demons. Nobody is sure whether Tieflings are Mist Demons who discovered a way to survive at higher altitudes or if they are a species created when some of the Mist Demons bred with humanity. But they came from the lower altitudes of some of the habitable plateaus and they seem to have a higher tolerance for The Mist.
Higher plateaus have thin air and are colder. This is were the Dwarves hailed from. Most of their people still live up there "In The Thin" but some have migrated down to lower altitudes where their stout frames, adapted for the cold, serve them with extra toughness. There are plateaus even higher than the Dwarves can tolerate and that is where the Angels (some say even the Gods) dwell.
Transportation between plateaus happens in a lot of different ways. Some are connected by narrow land "bridges". Some of these bridges dip down into The Mist or arch up into The Thin where only those who are tough or have magical or alchemical assistance dare travel. Caravans move along these paths where they are often in jeopardy of attack from barbarians, monsters, Mist Demons or Drakes.
In other places the way to get from one plateau to another is by flying. Sometimes this is done with mounts like Wyverns or tamed Drakes. Sometimes on the backs of the enormous Mist Whales. In other places the Eladrin use their Winged Ships to sail across these expanses. In any event, it is a risky, dangerous business where one might come under attack from Sky Pirates, dangerous creatures or even be forced by a storm to land upon a normally uninhabitable plateau.
For the Kingdoms where magic is more prevalent, the Wizard's Guild maintains strict control over trade and travel by use of their Linked Portals (as per the Ritual). They take a very dim view of those who try and circumvent their Portals and have been known to burn Airships and Mounts for daring to challenge their monopoly.
For the average peasant the whole of their lives goes by without them ever setting foot upon another plateau. But for others, they dream of what lies beyond the horizon where the cliffs fall away into The Mist. Intrigues and exploration may take them to foreign plateaus or even those that have never been discovered at all. Legends tell of places far, far to the west where there are gigantic plateaus the size of a hundred Kingdoms where one can walk for weeks without ever leaving the same plateau. Could those legends be true? Perhaps the PC's will find out.