There is already the donators sub-forum which is essentially dead and I can't see a Legendaries sub-forum going any different. What might be interesting is a sub-forum for default trust. There are often issues which require discussion about inclusion or exclusion and at the moment the only way to communicate is to PM 10 people and spam everyone's inboxes when anyone replies. A sub-forum may organise this differently.
I think thats a good idea. It should be read only to everyone though to keep things transparent.
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I think, this will be different because lots of Legendaries are still active as opposed to donators.
What would be legendary only topics though?
Other forums have this section or sub forum for Moderators and Admins where they can discuss what action needs to be taken and what are the thoughts of other Moderators and Admins and it's also a good section to police its own ranks. However, those section in other forum can only be visible to Moderators and Admins only to protect the them from members with issue and to avoid further drama.