Ralkage or just wait until someone extends the current tag extension
To achieve the desired effect, an extension programmer would just have to create a pseudo user group "original poster", the person that started a given discussion. Then the admin could limit the right to read and write posts in a specific tag to the OP pseudo group and the moderator group. As everyone is potentially an OP, everyone is allowed to enter that tag and start discussions, but they will only see the discussions they started themselves unless they are a moderator, who can see all discussions.
[...] and make an improvement to where tags can also be user group based.
Isn't that already the case?