As far as I know this isn’t a feature that exists, but I know the protocol should make it fairly easy.
What I’m thinking is for Lemmy to basically have the option to inherit the comment thread when posting a URL to another fediverse service.
E.g. If crossposting a Lemmy thread, you get a tickbox saying “inherit comment section” or something and it makes the new thread effectively a symlink to the original.
This could also be used to bootstrap other fediverse services like pixelfed and peertube by enabling people to comment directly from their Lemmy instance.
To deal with moderation conflicts you could basically just unlink the thread.
Also we could also just add flags to activitypub messages on creating posts to tell the boosting server whether or not merging is permitted.
If you’ve worked hard on something and want everyone to see it, set it to True and people can boost it to different communities.
If it’s something personal or an in-joke that you only want to be advertised to the community it’s posted in set it to False and the option won’t be available when cross posting in the UI.