I’m very new to this site so I’m not sure what all already exists. Some features that come to mind based on my experience on Reddit and other sites:

• Ability to search the entire site to see if a string of text (or multiple select strings of text) has already appeared there, including removed content. On Reddit, this was useful for seeing if an account has copied the comment, the text within a post, or the post title from elsewhere on the site. SocialGrep, Reveddit, and Unddit were my preferred sources of this info for Reddit. Text may also have been copied by a bot from other sites, but the original tends to be more accessible in those cases.
• Ability to search the entire site to see if an image has already appeared there. This was essentially only relevant for repost bots and for bots that recognize an image from another post and re-comment from that other post. I do have concerns about this becoming relevant in the future for comments that contain images. TinEye and reverse image search on Google were my preferred sources of this info, but I don’t know if Lemmy posts will show up on those sites. u/RepostSleuthBot and the like were also helpful, especially if summonable in the comments.
• Blocking users should only filter them from the blocker’s feed, rather than make the blocked user unable to comment on the blocker’s posts and comments. Spammers and scammers would abuse this system to prevent human users from calling them out on being spammers and scammers. While this design makes sense for sites based on personal profiles such as FaceBook or Twitter, it does not work for sites categorized by subject matter with impersonal user profiles.
• Say what you will about the bad aspects of 4chan (and you should!), but the use of Captchas prior to publishing a post or comment seems to majorly mitigate bot activity.
• This doesn’t seem to be a problem on Lemmy, but on Reddit, not all of the information of a spam report was sent to the subreddit mods. A report for Spam -> Harmful Bots would tell the admins that it was a Harmful Bots report, but the mods would only see it as a generic spam report and not be fully informed of the issue. Also, unbeknownst to mods, admins could link a subreddit rule report to a sitewide rule report. What I think Lemmy could improve on in this regard is to keep the openended custom report option, but also include pre-written report options for community rules, instance rules, and sitewide rules.
• Some sort of indicator for groups of accounts which seem to be commenting only on the exact same posts as each other, which commonly are bots.
• Entirely dependent on the subreddit or community, requiring some sort of verification post or other verification with a photograph of a paper with their username, the community name, and current date on it prior to permitting the user to post/comment may be beneficial.
• A sitewide blacklist structured like r/BotDefense, wherein suspect accounts can be submitted and, if determined to be bot, will be automatically blacklisted from participating communities. Blacklist appeals will also be essential just due to human error.