One discourse norm that I think is really important is that of having to either support or retract accusations if challenged. If you say something negative about another person, their work, etc. and they ask you to explain yourself, I believe you are compelled to either justify or retract your statement. This creates a strong barrier against unjustified attacks and gossip, while still allowing justified criticism.
Here are some examples of what this norm might look like in action:
1. Alice posts about her thoughts on an issue; Bob, who dislikes Alice, responds with snarky insults about Alice's motivations. Alice asks Bob to explain his accusations, and he doesn't do so or replies with more insults. Moderation intervenes against Bob.
2. Carol posts a brief comment saying a project is incompetent. Darryl replies asking her to provide more detail or retract. Carol links to a post that explains her critique in more detail.
3. Efren posts a statement criticizing an event that will soon be held. Faye asks Efren to back up his criticisms. Efren decides that his claim was actually more emotional and less grounded than he first thought, so he decides to retract his original statement.
Now, someone might ask "why try to make it more difficult to be critical of something?" The answer is that making fun of things is easy , and in general norms online often trend too much in the direction of low-content mockery rather than reasoned debate. Holding norms that require people to back up or retract controversial statements can be a good step away from that failure mode.
 Full disclosure: I wrote the linked post under my old username.