It would be neat if I could go to a special "add friends" page on LiveJournal that would let me add offsite journals.
You already can, with RSS feeds. With one step of abstraction away, people are "supposed" to include a special <link> tag (I think?) in their <head> that directs people to the RSS feed, so LJ could pull that. With another step of abstraction away, I could just type in "williamgibsonbooks.com", have it autograb a FOAF file that points at his weblog which points at his RSS.
Unfortunately, LJ generally has a policy of never retrieving remote pages in direct response to a user request (I guess it helps keep the possibility of "attacks" down). The one exception is userpics, though I guess syndicated accounts are another example. Also, FOAF isn't common enough for this to be useful, and would likely just cause more confusion because some (very rare) websites would "just work", while the rest would produce yet another broken syn feed.