It would be very cool if certain aspects of the social networks created by the different LJ-based sites could be absorbed together. Right now we have the rather lame solution of adding RSS feeds from other sites, which taken to the logical extreme means that all of the sites end up sharing one namespace, but all of the journals have different names depending on where you are. This is a mess. RSS works okay for pulling in content from outside, but when it comes to other LJ-based sites we could do so much better, and do it much more efficiently. I should be able to add firstname.lastname@example.org to my friends (or "watch"!) list on LiveJournal and have the entries from that journal appear on my friends page. We talked about this a few years back, but the situation has got a lot better since then as other people have done some of the things we would have had to do.
On the surface this doesn't seem too hard: we need some efficient mass-transfer protocol so that the sites can pull updates (create, edit, delete) in batches between each other, and some way to express what journals each site wants. Sending the list of journals over each time would be a bit lame, so each site could instead maintain subscription lists for the other sites it can link with. Of course, this requires co-operation between the different sites, so it's not brilliant. Also, I'm not entirely sure if we have the right data to be able to distinguish the create, edit and delete operations... but syncitems does this for a single journal, right? Since these special accounts won't have any of their own journal views, the entries can be safely deleted once they're too old to appear on a friends view.
By now the Atom folks might well have something we could use for this. Back when we originally discussed it I was pushing for creating our own XML format, but that was before the Atom folks came along and did basically what I was proposing. However, last I was tracking Atom they were still trying to decide on the XML format and not really near any kind of API for pushing content around in an organised fashion.
Obvious caveats: need to be able to comment on other sites with a LiveJournal account and vice-versa (decentralised TypeKey-style auth fixes this), handling security-locked entries securely (can't without an user-trusts-site relationship), knowing how many comments there are on the remote entry (or just make it say "Read Comments" for those)... and do we let (say) DeadJournal users post in LiveJournal communities? Should communities be shared too?
Yeah, this entry doesn't have any concrete stuff in it. I think it's worth working towards a proper design for this, though, as it's something we've talked about forever.