I like the idea of viewing multiple communities. However, I don’t like the idea of cross-posting to all communities at the same time - it would prevent communities with overlapping topics to diverge and specialise, since people would mostly post the same stuff in all of them.

Instead I think that a multireddits-like approach is better: you group comms for visualisation, and your group can be either private or public. If public, other people can copy your group, so they use it instead of subscribing to individual comms.