Been asking for sub categories to be added in the New Wix Blog for months now (they were in the Old Blog, so I am not sure why they were taken away when the New Blog was introduced), I have quite a lot of content, and would like my blog to be organized, functional, and accessible for readers so I really can't blog without them (literally still unable to publish my site at this point...). I love that the New Blog has a search bar, and more importantly a categories navigation bar - and it would be awesome if the sub categories could drop down when scrolling over the main categories.
I also really think each category and subcategory should have some kind of introduction pages/boxes when readers click on them. Maybe main categories can have a customizable introduction page including links/buttons to each subcategory, and each subcategory can have a customizable section/box above the blog posts that can include images, text, and/or icons (quotations, etc.) to build up the look and description of what the subcategory is about. I know Wix recently added a category description feature, which is great for SEO, but I think it also makes sense to allow for something similar to be allowed to be added on the category/subcateogry pages themselves too.