One potential PoC for a plug-in could be a “conversation listener”?
Something which notifies you whenever a new comment or reply is made to a selected post/comment, so that you can keep track of any new conversation.
Dunno how viable that sort of thing would be though … how could plugins affect the UI? Plus the plug-in would need its own database.
Beyond that, moderation seems like an obvious application?
Perhaps some sort of wiki like application, where posts containing certain tags or other markers get automatically organised someway in a separate post or page?