Do Holmes and Watson have to be white? busaikko has several Harry Potter stories in which characters whose ethnicity was never specified (and who we therefore expect to be white and British) are from Brazil or Japan or...wherever. The way she did this was by making them POC in Britain, which means knowing something about that minority experience. Or you could make Holmes Japanese, but that means knowing enough about Meiji period Japan to figure out who Japanese!Holmes was.

Well, except that he's in the Full Metal Alchemist parallel universe, which I don't know much about, and maybe Holmes has to be, whatever--Ametrisian?