When a discussion is interesting enough for me to want to follow it, I will search the page for "minutes ago" to find the newest posts. It might be nice if new posts were highlighted for some amount of time.
I have a user script for that [1]. If you visited a story's comments before, it will highlight new comments and can auto-collapse threads without new comments. Otherwise, it gives you a slider you can use to highlight the X most recent comments.
Maybe a dynamic system that randomly shows an order weighted on upvotes in the past X minutes. That’d give fresh comments a chance to compete.