Aliasing would work here. Allow a user to create an alias "meme" community that contains multiple meme based comunities. So when a user submits content to the alias the home server can just publish it on all communities. This is only user visible, so the community itself doesn't change... but from a user perspective you see more content under the same alias. Posts made this way could also have some additional Metadata to condense them together when you see the same post on multiple communities.