Not OP and not involved at all in the development of the fediverse. But this is how I would do it, and if someone gets inspiration from it feel free to use it.

Upon creating a post, unlike now, it wouldn't be created for a community. Instead posts would be created under an instance. Each instance would have its own rules about posts and the admins of an instance can always decide to remove/edit/hide/whatever the post from the whole instance. As a user of an instance I'd assume they should follow the rules entirely of that instance at any time they interact in it.

Each post then could have a list of communities it is posted to. A post with no community would be part of a kinda global no-community community with the instance name or something (a different instance would then see it as a community-less post from an instance and can show it just like that.

Each community would have its own mod team and rules. As a post doesn't belong in a community, mods cannot remove or edit the post. But if a post breaks rules of a community that are not rules of the instance (like an instance that allows nsfw but the community does not), the mods can choose to hide any post from the community, and maybe even control if the user can attach a post again to the community.

That would include communities in other instances, which would link to the original post to take into account changes and what not. But now, both admins and mods can only hide the post, from the whole instance or the community respectively.

Comments belong to the post, of course, but comments could have some user modifiable field to exactly say what community they saw the post in and browsing the comments would be allowed to filter by community, and just like now, comments need to follow the rules of the instance. Mods can choose to hide comments specifically but only mods in that server can remove the full comment