With IP.Downloads it would be nice to have an option to show the subcategory files in the main content area of an empty "parent" category. An example of what I'm talking about can be seen here:
If I were to link to that category at first glance it isn't obvious there are any files available as the subcategory menu is easy to miss unless you are used to the interface and the words "No files in this category yet" are much more prevalent.
One possible approach I've thought would work well for these types of "parent" categories, like how with IP.Board forums you can list it as either a category (no posts) or a discussion (allows posts), would be to show each of the sub categories and their top files in the main content area. Or another possibility could be to just show all the files from child categories the same way you would if you were viewing each child category individually.
Expanding on the usefulness of these "parent" categories could also include when you follow content in the parent it means you also follow all child categories as well. Or alternatively removing the follow button completely, like is done with forum categories, as it is misleading since no content will ever be posted in an empty parent category.