Get rid of the current moderators and pay professionals. Automate the universal rules and use these folks to steer content back to when it was most meaningful. Also dedicate as many resources as possible to identify and prevent astroturfing and shills.
Ok, one of my favorite subreddits is about an specific American book series based on a modern genre of Chinese fiction. Do you think that Reddit can scale to manage that kind of content without volunteer mods?
It's the same thing as people used to say about Wikipedia. Oh, they shouldn't take volunteers and just pay for encyclopedia editors. But if you want content at scale, you need to provide power to people so they can say what they want, and they are going to abuse it. But the alternative as never worked, and in fact, people get really mad here when people try to implement the alternative.
