Basically, I want URLs to be handles and self-hosted blogs to be posts. I'm still trying to figure out the federation aspect, but aggregators could scan domains for lists of users by checking "johndoe.name/microblog.json", or there could be a centralized server with a list of aggregators.
If microblog.json exists, it contains a list of users' URLs, and each of those is a list of posts. Each post has a short, less-than-140-character summary, and it may have a longer version. The aggregator can decide how to display short vs. long, but I'm imagining having the summary be clickable, and then the full text will roll into view.
The aggregation server otherwise functions a lot like Twitter. You can follow other people, and it might suggest other people to follow.
URLs are long, of course, but there are ways to shorten them. If the user is posting under their domain (johndoe.name), then their handle just becomes the domain.
If they're posting under a subdirectory-style URI, like "twitter.com/johndoe", then their subscriber only sees the "johndoe" part.
The subscriber doesn't need to know which domain "johndoe" is from, because she chose to subscribe to johndoe and will recognize the handle. If she subscribes to "someotherdomain.com/johndoe", then the aggregator can cleverly distinguish between them (with a tooltip, different colors, small subheading, etc.)
An initial aggregator would have to include Twitter, Google+, Tumblr, and any other publicly-hosted blogging service currently used for short posts. That would allow people a smooth transition.
