I would like to third the suggestion of using tags.
I’m a moderator on he NaNoWriMo forums which are an Enterprise customer due to the sheer size of the forums. We have over 900 categories and while necessary in our case, it makes moderation an outright nightmare at times because some places in the UI are glitchy because there are so many categories. As an example, the categories menu on the Create Topic screen takes anywhere from 1-5 seconds to open.