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.
There are pros and cons for it being public. For example discussing CiTM in the public before he was overthrown got you disavowed from DT and negative trusted. People can be unwilling to speak out at their own risk and also propagates people using smurfs.
Well CITM (and those in his trust list) would have been able to see threads about CITM, so having a private DT sub would not have changed this.
I think it would be bizarre to have a subforum dedicated for one user's (two if you include QS but he's not in DT) scheming.
Except that I have never left negative trust because someone was speaking out against me.