Blogs publish articles, or entries, which are basically the same as Usenet posts – but there’s no widespread mechanism for grouping and replying. We can fix that.
All reputable blog systems produce a URL that yields a full-text RSS feed. That takes care of publishing. Traditionally, Usenet articles have headers signalling From, Subject, Newsgroups, and References. From and Subject aren’t needed – the From is either in the URL or in the blog’s content, the Subject is in the title or body. Newsgroups is a super-threading semantic, and might not be needed. Tags could be a better arrangement, particularly if people agree on some distinct keywords when they feel it appropriate.
Then we add a threading semantic to the RSS feed signalling that a particular entry is a reply to a particular thread. One obvious mechanic for that is for all participating blogs to guarantee that the URL for each entry on their site is unique, and use that URL in a Reference header signalling that this entry is a reply to that particular URL. Let’s mandate the Tags header as well.
Now we need an indexing/discovery service. Let’s make sure that’s federated. Compatible servers can hook in as new leaves, getting a copy of all the known living feed URLs plus the last hundred tags seen from each URL. Note that we are indexing and searching on metadata, not data: some megacorp might want to suck down all the data, but a server should be runnable on a small budget by an individual.
Finally, ask RSS readers to fire off the blog-writing client/editor to construct an entry with a particular Reference header on demand, copying the Tags but leaving them editable.
Et voila, Super-Decentralized Usenet.