Maybe a partition, since one country getting all of Japan probably wouldn't be acceptable. So Russia gets Hokkaido, France gets Kyushu, and Britain takes Honshu. Not sure who might get Shikoku--maybe Portugal, the Netherlands, or Spain, although all three countries are probably incapable of holding it. If it's post-ACW, then maybe the United States might grab it but that's debateable too. Maybe either Britain, Russia, or France.
Alternatively, maybe it could be a protectorate in all but name like Thailand, with Japan consisting of only Honshu with the rest being given to other powers. It would be a British client state in that case. In return, Russia would probably be given much more free reign in Manchuria and Korea.