Categories certainly will be evolving. If you have a new category proposition, create a new voting, here on Meta. For example, "New category proposition: [name]" with options like "I support this idea/I oppose this idea". Then add a description of course.
As for the "tags system", you're referring to the rankings system. OpiWiki is a very young site and many ranks are yet to be created. If a ranking doesn't exist, you have to create it. More info: opi.wiki/help/ranking#c