Gotta say I like merged communities better than just multireddits. The problem we're trying to solve is that one community of 1000 people is more than 10x better than 10 communities with 100 people, because instead of a bunch of posts or comments with less than 5 upvotes you get true content curation.
Would have to be voluntary and maybe there could be two levels, one where mods can only mod what is "truly" posted to their instance, and another where any mod can moderate anything in the combined community.