I'm aware that Greek and Roman mythology is on topic.
But is Sumerian, Mesopotamian, Babylonian, Buddhist and other mythology on-topic as well?
Yes, there are no geographic restrictions on Mythology, in this proposed site.
I see no reason why they wouldn't be... Mythologies exist on a spectrum; there are rarely well-defined lines. If a question is asked about Astarte is it off-topic to discuss Ishtar? Where do we draw the line with Cybele?
Mythology.SE is going to be convoluted enough but I don't believe we should be placing restrictions on this sort of thing.