I can appreciate the unpredictability and potential layout difficulties of showing all categories and subcategories in categorybox. That's why I think a drop-down selector would be a good alternative. Something that would look like:

All (root)

- Top category 1

-- Subcategory 1.1

-- Subcategory 1.2

--- Subcategory 2.1

- Top category 2

-- Subcategory 2.1

You get the picture.
Yes, this list may in same cases become pretty long, but it may still be a good alternative to drilling into the current categories.

How about we make such a selector?

I tried but I am having a heck of a time generating the list of all categories and subcategories with available functions.