Communities on different instances about the same topic should have the option to essentially federate so a post on one appears on all of them and opening any of them shows you the comments from all of them. This way when lemmy.world is down its not a big deal because posting to any news community federates to all of the communities instead of barely having people see your post. Federation could be decided by the community mods and the comments can have a little “/c/communityname@instance.name” on it so you know which community the comment was originally posted on.
I’ve had that thought too- it would guarantee instance owners are dedicated to making one community as awesome as they can, but at the same time the current structure means non-technically inclined people are able to have a home off-Reddit as long as their values align with the instance owner.
That said, Startrek.website is kinda doing a focused-topic thing with different communities and rules within to achieve different goals working with the same subject matter. I think it could serve as a good model for themed instances.