Special thanks go out to Glenda Pfeiffer for this one, because she and I had a terrific one-on-one conversation about mythology, which covered some things you might expect and other things you probably didn't!
We started out by defining some of the conditions under which we've seen mythology used in worldbuilding. They looked something like this:
1. Building a 'real' world where an existing world mythology is true. This is the Rick Riordan approach, where you have Greek and Roman or Egyptian gods showing up in New York and other real world places. It also includes things like urban fantasy where our own world coexists with vampires or fairies etc.
2. Building a world internal to a mythological canon in order to extend it. This is what people do when they write "new myths" which fit into a particular paradigm. I think particularly of the way Hayao Miyazaki and Mizuki Shigeru have treated Japanese mythology in their work - and the way people like Howard Pyle and Theodora Goss (and others) write new European-style fairy tales.
3. Building a secondary world with its own mythology/mythologies.
This third one is where we put much of our attention. It's something that has also been done by a lot of authors - though it's worth asking what is the difference between creating a religion for a secondary world, and creating a mythology. In fact, mythology is a word that is often used to describe old religions that are no longer widely held - which means it can be offensive to label something as a mythology that is still an actively practiced religion. Mythologies also often contain accounts that sho the layering of historical belief systems, as in the Greek example where Zeus' many love affairs show the way that the belief in Zeus was able to incorporate local female deities into its stories as the religion moved into those regions (and conquered).
Glenda noted that mythology often contains elements of true history that have devolved into mythology over time - or, one might say, evolved into mythology. Elements of true events remain within a storytelling matrix, an archetypal scenario that departs from a precise historical account. Lost colony stories are examples of these; so are origin stories.
One thing you can do as an author is take inspiration from existing mythologies when designing religion or mythology for a secondary world. My own Varin world uses a religion that is very loosely based on Greek mythology, in that the gods are a family with representation among the heavenly bodies, and each one is the patron of something different. Varin also has a mythology of sorts - but this one is an ideological mythology concerning the origins of their society and the role of their hero, the Great Grobal Fyn, in its founding.
Glenda asked a question that neither of us knew the answer to (and maybe some of my readers do). She wondered whether there was a place for tales of genies within the context of Islam, and whether there is a historical connection of some kind.
Getting back to creating a mythology, the form a mythology takes is going to depend a lot on the setting where it exists. Do your people live near a volcano? A river? On an island in the ocean? Then those elements are doubtless going to appear within the local mythologies and origin stories.
You can also ask, "What is the place of the mythology in this culture?" Is it something quaint or "cool" that people recount stories about, or is it an active teaching tool? Is it a driver of actual behavior, like the stories of golden cities and Amazon women that sent explorers searching into the American continents? Is there a mythology that has grown up around specialized settings like the frontier, or the mountains where hermits and "mountain men" might live? Furthermore, any society can have more than one existing mythology. They can have an ancient mythology, as well as a current ideological mythology (like the Varin example above) and also urban mythology. In fact, I had never thought about what kind of urban mythology might exist in Varin, and after this discussion, I was inspired to give it some thought! I'm sure the undercaste characters who are the focus of my next book will have non-regularized urban myths that they believe in. (Exciting opportunity for me!)
At that point we talked a little about origin stories. Do the people in your world believe that a deity or deities created their world? How, and out of what? Was it a darkness-and-light thing, or is the world a deity in itself (herself)? Was the world made from the dead body of an evil giant? Was it a blanket that was woven by the gods? Were people made out of clay? We can take inspiration here from all kinds of existing world mythologies and religions. Greek, Norse, native American Indian, Semitic, Amazonian... the list is endless. I mentioned my own surprise upon reading an English translation of the Kojiki, which is the native Japanese origin story. In that story, the children of gods become islands, but all sorts of divine bodily fluids and excretions also turn into gods, or islands, or other elements of the world.
Another good question to ask involves the roles of animals in the world, and in its mythology and origins. Are they created alongside humans, or are they participating as deities and creative agents in their own right?
We spoke also about Watership Down by Richard Adams, which is a brilliant book that features the mythology of the rabbits as well as their society. It's a great example to look up for the way it features the natural environment and capabilities of rabbits and turns them into sources for teaching tales.
Glenda said that Feng Shui has taken on its own meaning across contexts. We both thought that directionality was very important in mythologies, including all those with personalities for winds from different directions, and those like the Japanese which included directional taboos (days on which you couldn't travel in a given direction). We wondered what a mythology based on an absolute direction system (rather than a relative directions system like left/right) would look like.
What kind of mythologies would develop in a spacefaring culture? Would there be mythologies of the cosmos?
Needless to say, we could only scratch the surface of this topic in the time we had. I hope we'll have a chance to discuss it again soon.