Idea: enable downvotes, but they don’t just lower the ranking of the comment you downvote, they lower the ranking of the top level post. And they have a much larger effect than upvotes on ranking.

Now if you get into a fight with someone in the comment section, nobody will see it because your downvotes on each other will push the whole post into obscurity. Only uncontroversial posts where everyone agrees succeed. Thus, people are incentivised to make posts that everyone can enjoy. If you post something racist and start a fight in the comments, your post gets no upvotes, and the fight is contained from spreading further.

Downside: this system doesn’t discriminate between content that controversial because it sucks and content that’s controversial but good. For example, any kind of vegan content is going to be buried. You’ll get a much stronger hivemind effect on this social media.