Content erosion and updated (cross-wiki) WikiProjects
I don't see how content errosion could be countered on Wikimedia projects where the main problem is the lack of editors at all though.
As for the later part I think better possibilities to cooperate across the borders would be very good. Woodwalker has made this point here earlier, but at the same time the usefulness of such coolaboration for the single editor was questioned. If there is a single editor that edits an article, what would make him put extra effort into channeling his experience to the large community? Maybe colaboration at a project containing a large amount of articles would be useful. Is this something that could be combined with the proposals of larger social networking possibilities? Giving editors not only a place to write articles about there subject of interest, but also to find others with the same interest that they can exhange ideas with. Maybe portals could have multilingual forum spaces and chat rooms.