On the top of my head, I’d say a trust-level system would be great, both for instances and users. New instances and users start out on a low trust level. Posts and commemts federated by them could be set to require approval or get deranked compared to other posts and comments.
Good thinking, but devil’s advocate here: might make it difficult for new users to post anything. I can imagine a lot of communities would utilise that feature, maybe even the majority.