Some interesting thoughts from
vizi on mythology in roleplaying. I agree that the treatment of most Religious/Mythic systems in RPGs is unsatisfactory. In constructing my own, I tend to focus in the region of my academic interests (theological systems, rituals, cult topology, and politico-religious interactions rather than mythology) but I'll have to think on incorporating the mythic past for my current game.
(
basal_surge's comment on realistic language also should be a consideration if you design worlds.)