A distributed message bus seems to be the most promising model. It'll be better than 'twitter but open' through the variety of messages being passed (see the sidebar of the UI mockup below). I believe the messages being spoken will come from activitystrea.ms, but the bus itself has yet to be determined.
There is some interesting work along these lines from the founder of StatusNet going on in https://github.com/evanp/activitypump
A UI mockup of a client that would read from the 'pump' is here: http://statusnetdev.net/inbox.html