A variation on this theme: make people frame their comments as falsifiable predictions. This is what Philip Tetlock advocates with his Superforecasting concept and sorta implemented here:
The hitch: making good falsifiable or verifiable predictions is tough and time-consuming. Evaluating and judging them requires even more moderation.
And let's face it: most online comments are not about advancing the public discourse. They're about getting a quick dopamine hit.