I don't really want to name this CategoriesInterface. The whole category system is a tree implementation, which is why I implemented a node interface here: #21327 I don't know how to name it in the tree-object-sense, but maybe use something that more aligns with tree terminology, instead of focusing heavily on the categories stuff. In an ideal world, JMenu would implement the same interface and menu items become a NodeInterface object, too.
I agree for the tree, but the object which returns the tree is category related. So it makes sense to name it CategoryInterface. The implementation class should probably being renamed.
I need to get this one merged to finish the service conversion of com_contact, com_newsfeeds and com_categories.