The surface world in my Dreaming Gods campaign will be known as Ezzin. The planet is very Earth-like with many races, cultures, and religions. It will be rather similar to a lot of fantasy settings, but I'll list a few of it's more unique quirks.
Because of the "portals" any character or creature might suddenly find themselves in a strange new land, but thankfully it is rare on the surface. They return with strange tales, and sometimes strange items. Some of these items have been given special religious meaning. Imagine a priest interpreting an Ipod with earbuds as a magical communication device full of "teachings" from Dio, the Lord of Metal.
The Dreaming Gods. Almost every religion on Ezzin incorporates the idea of dreaming gods in some form. This is used to explain the portals and reality shifts, they are the dreams of gods, or perhaps Ezzin is the dream. The details will be wildly different, but the basic concept will likely be present. It might be a single dreaming god, or many. There might also be a single, or pantheon of other (normal) deities. These deities might interact with the dreaming gods and mortals, or not. The important thing for me to keep in mind while DMing is that these deities will never physically appear, or provide more than circumstantial proof of their existence. The Azhuloughmahni (or whatever the locals call them) provide more tangible evidence for their existence, but they are also impossible to pin down physically. There are also whispered rumors of strange creatures or people appearing, birthed not of egg or mother but from the mind of the dreamers into the very air.
The creation story. Delve far enough back into any culture, and you'll find their story of how the world came to be. On Ezzin, these stories are all strikingly similar. Before time (500-2000years ago), Ezzin was ruled by beings (demons/angels/alien creatures) who call themselves the Azhuloughmahni. God(s) arrive and cause an event (the mutant apocalypse) that reshapes all the lands and creatures. The Azhuloughmahni were pulled from their bodies and forced to live as shadows within the bowels of the underworld. Most deep caves, and underground dungeons are thought to be the ruined cites and domains of the Azhuloughmahni. Some people are brave or desperate enough to seek the coins and treasures of this ruined civilization. The surface world was reshaped by "the Breath of God" or "The Waters of Change", and no trace of the Azhuloughmahni remains unless it is dug up or brought to the surface.