I've thought about making a forum where people can comment on threads from Reddit, HN, 4chan, etc... but I would have to reverse the moderation meaning a post is not visible until it has been reviewed and editing it would put it back into review state and nobody would like that.

The reason would be to have long running discussions which is the opposite of HN. Events evolve and so do discussions and details around them. Short lived threads also make it easier to memory-hole a topic.