I thing will be nice to have option to add descriptions to folder or file and this description to be shown as hint on mouse over and in small text area below the file/folder name.
You can already comment on files, would this be enough for your purpose? It should though be shown directly (create a feature request for that here: https://github.com/nextcloud/server/issues
Comments are not a proper replacement for real metadata. Furthermore, the comments were never fully implemented. Permissions and limitations are still missing today.