This is kind of world-building, kind of plot-building. I've established a world and mythology in which to set stories (including, eventually, non-mythology stories that take place in said world). I guess I need help developing my mythos a little more; I've got the creation myth and some other stories surrounding it, but I don't want to get trope-y (well, it is mythology). What kinds of stories do most cultures develop in their mythologies? Sorry for that rambling block of text. Any tips when writing in a mythological context? Thank you.
This is an extremely vague question. There's a huge variety of cultures, and a huge variety of myths within each one. Try Google?
There are quite a lot of myths for varying reasons. However, it's a pretty safe bet to come up with myths that explain things that can't be easily explained as those are present in a large number of mythologies. Examples: Rain, floods, tides, drought, animals, sun rise/sun set, stars, rivers, oceans, etc. Obviously there are more. Another way to think about it would be to come up with things that would be important to the world you've created. I haven't any idea what your world entails, but if you were writing a story about people who were at one time heavily dependent on agriculture as opposed to hunting/gathering, myths would include drought, rain, rivers, the sun, etc. Hope this helps.