I was also thinking about this recently, it could be useful if HN borrowed some features from mainstream social media forums, in terms of community cohesion.
For example, being able to award/react to comments like Reddit, Facebook and Slashdot have. Sometimes it's good to just get a feel for what others are thinking of a comment, rather than having to read a comment which either basically repeats what the previous commenter said but with different words, or just disagrees without adding much. I think this would improve the information density of the forum by encouraging substantive comments, and replacing the useless padding comments to mere annotations.
(Perhaps this would be a good use of emojis? Keep them blocked from appearing in the comments themselves, but allow them as short annotations to other comments.)
Another example, having a comment thread on each user profile, which could have a mix of public or private comments. Sort of like Reddit's messaging, but with a public aspect too. HN currently has no mechanism for direct messaging for some reason, and it seems a glaring omission, considering the community-building advantages this can inspire. This would fill this gap nicely.
One more suggestion: being able to add a photograph to your own profile. It's great to talk to other people in cyberspace with text, but sometimes it's also nice to get a visual indicator of who you're communicating with. This doesn't have to be a real life photograph of course, any representative avatar would be fine. I think it would add colour and warmth to interactions on HN. Perhaps to make it more interesting for HN clientele, it could be limited to something like monochrome 256x256 for that old school feeling, or you need to upload a LISP program to generate it.
Anyway just some suggestions, would be interested to know other HNers' thoughts.