My advice for creating a system of myths would look something like this:
1. What religions, beliefs, etc. have you already laid out? This might determine the theme of the myths. For an abstract example, in a religion with good and evil, moralistic fables might be common, or mythical creatures might be fearful demons or, conversely, benefactors.
2. What is familiar to people, and assumed to be a given? The people might assume the characters in their myths are like them, or they may be set apart by the fact they don't do something.
3. What is scary/mysterious/unknown/unlucky in this culture? Some myths might stem from that. Also, many, many myths serve to explain what science can't. If the people don't know about something, they'll assume whatever fits for them.
4. Think of cultural traditions and manners. Might myths factor in to this? In Ancient Greece, the gods could disguise themselves as humans, so people were hospitable to strangers because you could never tell if they were a deity in disguise.
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What creatures are there in your world? Are they a danger? Do they provide people with something they need? What do they mean to the people? Predators are often seen as fierce, and can be either wicked or noble. Animals the people need to survive often represent fertility. How your world views animals can affect legends and myths.
6. What are the morals of these people? Ultimately, this decides how the stories are told. What qualities are praised? What is taboo? How rigid is the society's power structure? The last one is important in that a rigid society might have stories with the connotation of "know your place," while a society with more opportunities might have a few "rags to riches" myths.
7. What aesthetic do you want your myths to have? Myths can be biased. They can praise, instruct, or simply entertain. Myths can be full of imagery and detail, or stripped down to the bare bones of its intention. What time period was this myth made? How did it come to be? How did it evolve over time? Was the meaning changed to suit society? Where parts taken out because of taboo, or something else? Another part of this is figuring out how symbolic you want the myths to be. They can be legends of everyday heroes, or they can be seeped in the culture, assumptions, and associations of the people so much that a foreigner would scratch their head at the meaning.
Hope this helps! 547