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?