I think we need a hard separation between topic tags and user tags. We don’t want to tag a user with theme-component or pr-welcome, for example… because it’s meaningless. The /tags page also needs to make sense, so mixing topic and user tags together there would be wrong. Most of the work to add user tags would be new work (UI, data models).
I think most people will want to use user tags in the same way we use groups (mentions, restricted categories, etc.), so maybe @bts is right that this is really a request to enhance groups.