I think most people were expecting or hoping for Aztecs or another mythology from the Americas. A lot of people were also hoping for a Mesopotamian (or neighbouring area) mythology, or for Celtic myths.

Personally, I count myself in that group. Aztecs would have been my favourite choice. However, if this is just the 2nd of multiple DLC pantheons we get, Japanese is a great choice. Just as the OG game gave us a bunch of civs within Europe and the Mediterranean, where they could naturally interact with each other without stretching credulity too much, going to Japan immediately after China feels like a good fit.

If, and only if, getting Japan doesn't mean that we never get the Aztecs, I'm really happy with this choice. But if this is the last big expansion we ever get, it's a little disappointing.