I have tons of icons in different categories. Using just a single tabset is not really practical for my needs. So it would be awesome if support could be added for subtabs. Either you understand what I mean right away, or you're totally lost now (what the h**k am I on about???). IF you get it, fine. Don't read on. The rest of this post is just a clarification. 
I have a feeling this really would benefit from some images, but I'll try to explain in words, as I don't have the proper software for design installed at the moment.
Currently the default tabs ends up "outside" the dock area (which some skins circumvent creating "integrated" tabs). It would be insanely sweet to be able to create subtabs that integrate into the dockarea, and have main tabs on the outside. My Games tab could have subtabs Online/Offline or my Media tab could have Playing/Editing/Burning subtabs.
I hope my general idea is somewhat clear, it would make the object dock the main dock on the block... Ehm, well, it already is, but it would help keep it there. 