Why? We could have something federated where I add my response to a particular post/thread to my RSS feed. We then leave it up to the client to generate a proper tree representation of a discussion.
Mastodon uses an HTML microformat called h-feed for federation of user feeds. h-feed is based on hAtom, which is a microformat encoding of the Atom feed format. So your idea isn't all that far off from how the most popular federated social network system is designed (though there's more to it).
