To clarify about hidden categories, hidden categories are currently used for categories like "Characters in..." and "Locations in..." in order to make the categories section a bit clearer, while still classifying articles under those categories. (Though, note that any user can choose to see hidden categories by enabling that option in their preferences.) I think using hidden categories would discourage new editors from incorrectly just using these new categories to categorize their new articles, so they would instead (hopefully) follow the correct standards of categorizing articles.
I think adding these type of new categories along with achievements for them could be interesting, however I'm not sure if it should be done.
What do you guys think?
Sounds good.