For example, if we just have Harry Potter as one category, people can avoid the characters they don’t want and DNW Pottermore and JKR tweets etc.

For what it's worth, I would strongly prefer not to see those sorts of DNW for this exchange; one of the main points of organising the tag set in this rather unusual way is to allow people to actively match on both the requesting and offering side with people who are on the same page as them about which bits of canon they're considering.