Please be patient, the community isn't always around and we got to sleep and work etc.
If I understand this correctly, you want your users to have the ability to create a tag for a discussion themselves?
As far as I know, this is not possible in Flarum and I've seen no extension do that yet. Developing one doesn't seem to be too hard though, it could just extend the Tags extension, without any extra settings (like permissions, secondary tags, etc. - just a plain label). The tag would be stored in additional column in the database - user_tag
. Possibly, you could filter certain phrases in user tags, or view all user tags in one place for moderation. That's just my idea
Overall, a cool idea and I like it. If I knew PHP, I would develop it :)