The Rules of Planescape

Back in the day, I had a fun time playing with friends in the Planescape campaign. It was literally an infinite tablet of interesting places to visit, with as much expanse and complexity as the DM wanted to put into it. For the uninitiated, Planescape as a campaign setting re-introduced the idea of an entire cosmology to Dungeons & Dragons. The game's setting could now traverse multiple planes of existence, truly breaking the traditional mold of standard fantasy settings. The story's characters would traverse different worlds via gates or dimensional portals or powerful magic of their own.

To create a unifying flavor for the setting, there was a lot of artwork done with a thread of similar style. And the writing and language was tied together nicely with easy-to-use slang terms invented for the setting. In addition, there were general some big philosophical rules that helped flesh out the place. Here's some more detail quoted from the Wikipedia entry for Planescape.

There are three principles (or heuristics) governing the world of Planescape: the Rule-of-Three, the Unity of Rings, and the Center of the Multiverse.

Rule-of-Three

The first principle, the Rule-of-Three, says simply that things tend to happen in threes. The principles which govern the planes are themselves subject to this rule.

Unity of Rings

The second principle is the Unity of Rings, and notes that many things on the planes are circular, coming back around to where they started. This is true geographically as well as philosophically.

Center of All

The third principle (fitting neatly into the Rule-of-Three above) is the Center of All, and states that there is a center of everything — or, rather, wherever a person happens to be is the center of the multiverse... from their own perspective, at least. As most planes are functionally infinite, disproving anyone's centricity would be impossible. In Planescape, this is meant philosophically just as much as it is meant in terms of multiversal geography.

Wikipedia entry for Planescape

My main character in Planescape was an arrogant Fire Genasi fighter/wizard nicknamed Koal. Koal was always on a serious mission of some kind, and never spent a single moment to stop and "smell the roses" in the Planescape scenery, but still managed to stumble around to interesting places.