How about Wordpress? What you're describing sounds to me like the standard structure of a blog with comments.

Twitter is a weird sort of blog platform.

It is chaotic by design: the chaos is consequence of its distinguishing features. Everyone does get their own 'mini HN', i.e. a forum. But they lack the tools to rule over that forum. It's idealistically anarchic. Everyone's expected to get along peacefully, totally in public. There's no ownership over threads.