Here's an interesting idea: Sub Categories. A sub-category would basically be a category inside a category.
Thanks for listening, Simple Machines.
I'm not sure what benefit this would bring, given that you can easily arrange boards inside boards...
I would say this has a benefit... For sites like my forum that would like to have 1 main category (say a manufactuer) then a category under that for each device that manufacturer produces.
however, that is a very specific niche.... I can't see this being really useful in the normal forum. Especially since, as Incognito points out, you can arrange child boards...