To make even better use of categorisation and allow users to find wikis via WikiDiscover, I think it would be worth adding subcategories to the existing rather broad categories.
Ideally, a wiki could still choose not to select a subcategory and just stick with the main one if they wish.
@Agent_Isai Could you start a discussion on Meta about this?
In addition, I also think since subcategories could be quite numerous we'd need to figure out a process of how (and who) ultimately decides what subcategories will exist for each main one.