I know LJ offers communities, but they are de-centralized and don't really unify the LJ population as a whole.
What if the community idea was twisted around to more closely resemble a bulletin board? There would be two types: 5-10 generic LJ-created forums, where anyone can post. In addidtion, there could be an unlimited number of user-created communities.
The stock communities would be "owned" by certain members of lj_support staff, so they could be monitored. Everyone could post new threads (journal entries, whatever...) by default, and would have the same powers they normally have over their posts *except* no deleting of other users' comments. That power would be reserved for the moderators (lj_support staff.) The main forums would have a front page for easy access. Threads would show up by title, poster, and datetime of most recent comment, like most BBs.
"Personal" communities, on the other hand, would be administered as they are now. The creator/owner is responsible for the day-to-day operations, and things would continue to run about like now. Communities (ahem, personal boards) would be categorized by their creators, and cataloged into a directory so it is easy for a person to find a community if they're looking for something. (As it is now, many people create a community, then realize five days later there is a similar community already).
A lot of this isn't as much work as it may sound, since the back-end already exists. The main issues would be little details of a forum, and creating the interface. I think a public forum that is easy to access and presented to users right off the bat would be a great think for building community amongst LJ users. Furthermore, a BB is much better at meeting certain needs, such as those of a business. It could help LJ gain acceptance.
This isn't totally fleshed out by any means, I want input! Comments/suggestions/death threats, I want them all!