However, if you choose to go that route, allow me to set an expectation or three.
Adding a subcategory is not always straightforward.
Under certain circumstances, an editor can add a subcategory based upon their own judgement, and upon the existance of a template that defines certain standard subcategories that can simply be added once there are enough sites to populate that category. For example, if I were to identify a handful of automobile dealers in anytown, USA, I could add the Automotive subcategory to the Business_and_Economy category within that locality because it is a standard category.
In some cases, editors are strongly discouragte from adding any subcategories, even if they seem straightforward. For example, if the major category is undergoing an approved reorganization (a process that can easily take a year), editors are discouraged from adding subcategories while the reorg is in process. Heck, in some cases, suggestions are even shut off in the affected categories.
Sometimes an editor will propose adding a given subcateory, based upon looking at topic area witha very narrow focus. That proposal may end up involving dozens of editors from multiple parts of the directory, all of whom wish to weigh in with the view from their respective positions.
So, why am I telling you this? Because I don't want you to come away with the idea that if you suggest a subcategory when making URL suggestion, that it is a done deal, and only needs an editor to simply create the subcat and shove your suggestion in there.