This thread is a little old, but I would like to express my support for the idea of allowing grouping as shown in the Diablo forums above as part of the standard delivery.
If I’m not mistaken, this theme component gets you close, but it would visually still be quite a bit different from the Diablo example. Also, it would be useful if such groups of categories also showed up in other places. Most notably, the list of categories on the main page and the side bar.
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