Between the worlds, lies Gyre.
The great cycle brings each of the worlds in line with Gyre. When a world is aligned, the gates of Gyre open to it. It is said that it is through such traffic that the worlds first diverged from each other.
The Lords of Gyre control the cycle and the gates. Some say they control the twelve worlds themselves.
The people of Gyre reflect the diversity of the twelve worlds. Many are as human as you or I. Others are Neanderthals, elves, cyborgs, talking animals, and even stranger things. Cultures lost on the twelve worlds might still survive in Gyre, and there are certainly cultures in Gyre that have evolved separately from those on the twelve worlds.
Within Gyre, culture and belief have power. When a person passes through a gate to Gyre, their bond to their own world is fractured, opening them to a possibility to wield that power. This ability can be diminished if they leave Gyre for one of the worlds, but this is unpredictable. Some people have left Gyre and imposed their will upon some of the twelve worlds. The eighth world, for instance, is ruled by an oligarchy of mage-kings who come from Gyre. Their infighting has largely torn that world apart.
When the great cycle passes through all twelve worlds, there is a thirteenth point on the cycle. During this time, all gates are closed and none can leave Gyre. Still, sometimes, there are those who pass through the seemingly-closed gates into Gyre during this time. Some of these seem to come from the twelve worlds. Others seem to come from somewhere else... perhaps other worlds that never fully align with Gyre.
Many things are said of those who pass through into Gyre during this thirteenth period. What is known, about them is little, but all agree that they have been marked by fate and have the power to change Gyre itself...
* * *
This might be the setting/premise for the next game I run. The PCs would be new to Gyre, entering during the thirteenth period. Various factions in the city might hunt them, seek them out for help, or hope to use them as pawns. In the meantime, they would learn about their potential, explore the city, and choose their own allies and enemies.
There are three main inspirations here. The first, and probably most obvious, is Sigil (Planescape). I'm not using anything like the D&D cosmology, but a city filled with gates to strange worlds is pretty hard to beat as a setting. I'm also influenced here by the whole philosophy-as-magic thing, though that's going to get filtered through the second major influence, which is Unknown Armies. I see a lot of conceptual connections between Unknown Armies and Planescape (and once wrote up a weird amalgam of the two). I'm likely to use Unknown Armies as the basis for the rules for this game, though there will be a few tweaks to magic (mostly to make it more flexible)... and combat. The combat system in Unknown Armies is, as far as I can tell, intentionally bad. The third major influence is Don't Rest Your Head. I'm not necessarily going for the ultra-surreal-and-frenetic nature of that game, but I am certainly influenced by it here. The PCs are newcomers into a strange world tangentially connected to their own where they are powerful and special... and the powers that be in that world will certainly take an interest in them.