An extremist discussion board, say dedicated to harming some other group. They may have no interest in advertising their plans, their speech is used to coordinate attacks.
I think that counts as conspiracy to commit a crime, so it's already covered in the criminal code? I'm focused on trying to develop speech norms for speech that's not a crime. See https://news.ycombinator.com/item?id=33998327
Also: libel. It does harm, to the point that damages can be awarded, but it's not necessarily meant to intimidate the target, just to misinform others about the target.
