Such a feature would be the only missing piece to use a category with default notification watch-first-post as an interactive newsletter that allows subscribers to quote easily and interact with other subscribers in the replies to that newsletter topic.
The only hurdle is the subscription process. Ideally, people would click a link, sign in/register with few clicks and confirm than the notification level change to x for cat/group/tag/topic y with x and y being specified in the link.
To draw even more attention to this feature, I will finish this post with a nice gif.