An idea I posted to the LJ Suggestions community:
My Live Journal wish: Cross Posting. I wish you could post one entry to multiple communities (i.e., the 23 million photography communities) and if you were a member of several of these communities, it would be only show you the post once. And, as all these communities would be looking at the same post, all the comments from the various communities would go into the single post.
It wouldn't be hard. Each journal or community refererences a uniq entry number for each post. There is no reason several communities can not refference the same community number. In the articles entry in the DB is has a field for this community it is posted in. Make that field much bigger and allow several forum id numbers to be entered, probably comma seperated.
The hardest part is finding any easy way to have the web or gui interface allow you to add multiple communities and journals. But if you decide on a limit to how many communities you can cross post to, then you can have that many extra fields on the advanced form/gui.
I love seeing peoples post, but I don't enjoy seeing it four times in a row.