One thing that could help improve this is better first-class support for cross-posting, integrations such as selectively merging discussion threads from communities on the same instance, to avoid fragmenting discussions, e.g. like how Hacker News can merge topics that reach mega-thread status. Posters could choose or be force to enable such cross posting for certain communities. E.g. if you want to post to the general !programming community, you first have to put forth a little effort to categorize your post by cross-posting from the most relevant community first. Although, this could be more akin to #tags, that may scale better with multiple matching categories.

Another feature that could help would be more advance moderation tools, such as post migration. Instead of closing or deleting offtopic posts, moderators could migrate posts with existing discussion threads to more appropriate communities on the same instance, e.g. just like how Discourse can re-categorize and manage user posts. I’m not sure how the logistics like the handoff between communities could work or how votes should be retained, but it’s just a thought.