What should Mythology be about? How can we make sure it stays on topic?
According to Alan Dundes, a myth is a sacred narrative explaining how the world and humankind assumed their present form
so it should be about any such sacred narrative, no matter if it belongs to dead cult or to the current world religion.
On topic should be questions about the plot of the myths, as well as the questions about their origins, range, local variations, available written sources etc.
This site should cover the histories, stories, oral and written traditions etc that have helped shape the world be it at a local folklore level(Haunted castles maybe), cultural history level (Greek and Roman Mythology), or global phenomenon (Sacrificial hero) level.
The types of topics (obviously not an exhaustive list) I can see fitting in well would include:
I am sure there is a whole lot more.
This site in my mind would include a discussion on Joseph Campbell's "A Hero with a Thousand Faces", a comparison of historic war deities, a discussion on human involvements in Greek myths (heroes), lineages, maybe a discussion on what the more mortalized (greek and roman for example) deities says about humanity.
Could be very interesting.
As far as staying on topic goes...this site would in my mind have to be far more conversation/discussion oriented than say superuser or stackoverflow, specific questions would of course be welcome as well.