Hello!
It might be great to have an option to mark category with 'always show', even if there is no items from such category in my bag.
Why do I want it? I have cetegory for each spec, and I tends to always keep my off-spec items in bank, so there is usually no items for Elemental when I play Enchasment Shaman. But, during lvling, dungeong, or whatever, I meet some new items which I want to keep for my off-spec. And I would like to put them in 'spec-category' right now, to not accidenatly sell them.
Right now, there is only one way to put items in empty category: Open options->find category->move item. Which is not really comfortable to do on regular basis.

There's already an easy way to do this (the dropdown menu shown when pressing Alt allows you to quickly assign items to preexisting sections):