One of the core things about HN sorting is penalizing articles that generate more comments than upvotes. IMO this is wildly successful.
To my mind the logical extension of this would be prioritizing comments that get more upvotes than responses. Those that fall below the threshhold would be sorted lower or default collapsed.
One cool thing about this is it could be a per-user setting, giving users a chance to explore how it would shift the conversation.
I suppose the way to evaluate this proposal would be to query messages from the last day that have 2x more comments than upvotes (for example) and observing their quality.
The second-order effect of this (server side) could be that suppose an article has 100 likes, and 100 comments, but 99 of those are sub-comments are arguing over a egregious 1-comment on the article itself. This 1-comment could be default-collapsed and the article could be treated as 100 likes, 0-comments. This would have the effect of deincentivizing irrelevant hot-button tangents themselves without killing the pieces of the discussion that are more fruitful. This evaluation would work from conversational leaf up to trunk, as soon as a score falls below threshhold that branch is "pruned" from the score.
HN is still working on rolling out some basic usability features such as pagination. Not to shoot down your idea, but realistically stuff like this won't make it into HN for a long time. And that might be a good thing - HN prides itself on relatively simple UX and fairly straightforward rules for ranking articles and comments (even though behind the scenes there is more sophistication, e.g. voting ring detection, etc).