You already can create JS plugins for HN with Greasemonkey.
I have one which currently:
- hides links based on domain (like techcrunch.com)
- hides links based on words appearing in the title (like gogaruco)
- highlights names of known posters and adds a string describing them ("somenick" becomes "somenick: Firstname Lastname (Companyname)")
- makes comment boxes bigger
- adds a margin at the bottom
I have one which currently:
It's pretty easy to do, really.