Correct me if this is already a thing, but it would be nice if you could post to multiple communities at once and have users see comments across all communities and instances. So a user posts “A” on instances X, Y and Z all under communities run on those instances at the same time. When making the post, you select ehich communities the post goes to instead of just one. Users on instances X, Y and Z see it as a single post it appears in all of the communities the user specifies. A limit might be useful here to prevent trial spam. A user commenting on the post in instance X will be seen on the other instances and communities where that post was made.That way, you could remove the centralisation on instances and communities (one community or instance might remove the thread, but everybody else still sees it and each others comments in the remaining communities/instances.) This has a few advantages:

• People are incentivised to post to smaller communities knowing that larger ones will also get the same post and everybody can see each others comments.
• If a moderator of a community removes the post, it still disappears in their community, but not the whole instance. If the thread still exists in other communities in the same instance, users of that instance can still participate in the post on those communities.
• If the post is banned instance-wide, it is banned across all communities in the instance at once. This could include non-local communities.
• Users in other instances will still be able to see the post and continue contributing to it. You can only remove the post from your own instance.