The mythos is a source of conflict in my stories, since celestial beings fighting between each other and humanity dealing with the aftermath is the core of the story. So for me, it's extra imperative that I focus on it as a primary worldbuilding thing.
This makes me go out of my way to answer a lot of different questions:
- What is heaven like, and how do angels function on a day-to-day basis
- What is hell like, and how do demons go about their business
- How do the souls of the dead get processed, and who handles it
- What is the presence of said celestial entities? Does anybody know they exist or is it a select few?
- How does magic function in the world I'm making? Who can use it?
These questions and more helped helped me build the foundation of how my story works, since at one point all of these become plot points I can use for character-driven drama.
It also affects my decisions regarding world design.
Example: The storm country has a lot of wind and electricity-based technology due to the natives being able to generate their own power. (It's also based off of Japan, so there will be advanced robot nonsense down the line.)
I second this. When in doubt, start with a celestial conflict and use it to explain the foundation of reality.
And once you finish developing the celestial world and need to build an actual world from scratch, I love map generators like this one:
Web application generating interactive and customizable maps
azgaar.github.io
Of relevance to this topic, this can generate a whole bunch of countries, geography, etc., and even give their political relationships to each other. It's an awesome springboard for inspiration. For example, maybe a mountain goddess somewhere offers blessings to travelers, but the nearby countries start a war to secure the mountain and her blessings for themselves. You look at the map to find a mountain and get an impression of local alliances, then you develop more details by yourself as needed.
After all, it's not just the celestial world that plays a role in developing mythology. Countries are the world's movers and shakers, and their actions pave the way for our futures and relationships (e.g., we won, so the mountain is in our domain, outsiders like merchants need to compensate us if they hope to pass by and meet the goddess, and we better make sure the goddess likes us so that we get even more blessings).
We could drill even deeper and start looking at the communities and individuals who power the country. Famous figures become immortalized for their accomplishments, but people don't become famous in a vacuum. They lead with the support of their people... but some would've liked that support for themselves because of various reasons. Groups form to support their collective interests: factions!
No country/community is ever a monolith hivemind.
They all have their own beliefs regarding the best way to go about business, and it would really help if their interests prevailed to become the country's interests. Factions develop based on aligned interests and compatibilities for limited resources (e.g., we're the eastern lumberjacks near the mountain, so screw the eastern druids because we need to trade wood and prop our economy vs. we're the eastern druids near the mountain, but the infidel lumberjacks risk angering the goddess by trampling over local property). The winner gets to become "mythologized" (e.g., King Greatwood negotiated a deal between the lumberjacks and druids - however messy it was - and with a mutually agreed-upon agenda, he had the opportunity to strengthen the kingdom by focusing on negotiating and leading a coalition with other countries).
As a side note, the further away in time from the event origin, the simpler the myth should be, since details get lost to time (e.g., the kingdom won because the goddess naturally chose the holy King Greatwood). You can then use minor details to flavor your story (e.g., Greatwood Day is a national holiday commemorating the day when the mountain and the goddess firmly became part of the kingdom). But if the story is set closer to the events and related to it, this is, of course, mandatory backstory to keep in the back of your mind.
When in doubt, create conflict at the highest level, then drill deeper as necessary, using whatever tools you need.