One thing that could be nice is giving established users an occasional nudge to have a look at /new (and a quick explanation of why it's important people do go there). Like, maybe if they haven't been to /new in a week, show a little banner suggesting they give it a go. If you wanted to get really fancy, you could even try to throttle the nudges according to the amount of posts being submitted at the time, so you drive more eyeballs to new when there's more stuff to be reviewed.
I know it took a long time for me to start looking at new, a little nudge would have made me start much earlier.
> We tried randomly mixing in new stories onto the front page
When you mixed new stories onto the front page, were they clearly marked as new or could they easily be mistaken for upvoted content?
Maybe this could be a karma gated opt-in feature that individual users can enable?
IIRC, the second-chance pool was our next experiment after that one. It has worked much better. The difference is human judgment or, if you will, taste.
It's not easy to come up with new mechanisms that might help with this problem. Every software mechanism we've tried allows many things through that don't pass muster. Community mechanisms, as soon as you open them up, get overwhelmed by people trying to game them to promote their own stuff. There's a feedback loop with that: the more interesting HN is, the more attractive it is to a high-quality audience, and therefore the more attractive a target it becomes for manipulation, which makes the site less interesting again. So there's a cap on how good it can ever get (https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...).