This got me thinking. If we all did this, it would be sorta like a huge online set of party conversations.
Imagine we each had a personal "party talk" email list. To my personal list I post all my "party" comments, and also appearing there are the contributions of others to the conversations these comments are part of. The readers of the list are as if they were standing next to me at a party, listening to what I say and hear. The comments of a group of us having a conversation would appear on all the personal lists of the people in the conversation. As with a party, you'd like some limited ability to move to a corner to avoid people you don't want around listening or butting in.
To implement this, one possibility is to let each person control who can read or post to their personal list. Then to listen to a conversation, you'd need the permission of at least one participant. And to fully join a conversation, you'd need permission to post to all the lists of the people presently in the conversation.
If a bunch of people all want to discuss a particular topic, they're probably better off creating a particular list for that conversation. But this approach might allow some interesting freer form discussion.
I suspect that managing email lists hasn't been made easy enough yet for most email readers to manage their own list. Especially with the rate of permission giving and revoking I suspect would be ideal for such a system. But we aren't very far away from where this is feasible.