Other forums solve this problem in the following way: they create a closed category which is only available to registered users. In this category people can discuss anything they want, but it won’t attract too many observers. One can call it “Round Table” as Purism forums did.
Welcome to the Round Table! This forum category is intended for certain types of topics that are either non-technical, highly subjective, or otherwise don’t fit well within other existing categories. This might also include general discussions about operations, philosophy, business, etc. Heated discussions may sometimes occur with strong opinions, but remember to keep it civil and constructive. Assume people mean well, are doing what they can, and that insights and information may take time to emerge.