Create a category say “feedback”.
Create a Post in category feedback, have it say something like this:
Presently the category heirachy is:
Dogs
German shepherds
Other breeds
Cats
Tabby
Persian
Everything else
If you would like to add additional categories or change this arrangement then please reply to this post.
Then give this category its own menu entry, and wait for the requests to come in.
For the sake of presenting alternatives (Ross’ is a good idea), you could utilize tags if they are not currently being used, or another added taxonomy. The taxonomy would not display on the front end, it’s solely for suggesting categories. An admin can go through added terms once in a while and decide if any should be added to categories. They could then apply a meta value indicating the term had been reviewed. Thus admins only need to review new terms without the meta value. Rejected terms remain so users can see they have already been suggested.
This way, authors can suggest terms right on the post edit screen, they don’t need to go elsewhere. An embellishment would be to remove the default term meta box from the edit screen and replace it with a custom box that is labeled more clearly that terms added here are suggestions and will not appear on the front end.
