Your proposed tagging system exists; it’s a curated folksonomy, used by the Archive of Our Own https://archiveofourown.org/faq/tags?language_id=en to good effect. One thing to consider: you are suggesting what is likely to be an unnecessary amount of top-down control. It turns out that users are pretty good at tagging useful things, especially if there is — as you suggest — a top layer of prepopulated tags. If tags are confined to a filterable list/area, then it doesn’t matter if there are thousands of useless or redundant tags — it’s not as if the library gets full and other things have to be sacrificed to preserve the tags— and things that might not seem useful to you or me can seem very useful to others, at least as a signal. See, e.g., https://archiveofourown.org/tags/I'm%20Bad%20At%20Tagging Promoting a tag to “canonical” when it proves popular enough can be useful, but even without that, folksonomy is a really good way for a knowledge community to improve its connections.