A new category is only created when there are enough sites to put in to it and the main category has become too large and needs splitting up. Sometimes if a category is split up, some of the smaller ones only have a few sites but they are needed to make the organisation logical.
So one way to suggest a new category is to suggest enough sites to an existing category with a note such as [?new sub-category <name>]. When an editor reviews the sites, they will then think about it and may or may not create it.
I shall leave the question of suggesting a new category so you can apply to be an editor of it, to the more experienced editors here.
regards