It depends on how federated platforms react, it’s necessary to control who signs up in some way, if that’s “globally” accepted trust can hold well long term.
Right now the instances are working to stifle this by restricting account creation, but we're just a step away from spammers creating instances on demand, flooding networks with stuff like the crypto spam on Reddit. I'm thinking major instances are going to have to go whitelist federation as a result.
I would not be surprised if at some point you have a few whitelists, or some kind of reputation management for instances. One could even say they can have a karma number associated to them.
In the end, this is the same problem as for emails and the landscape will probably structure itself around several big actors and countless of smaller actors who will have to be really careful in order to not be defederated.
That's an interesting idea. For each instance give users the ability to mark as spam comments/posts, then make it so each instance keeps track of what the ratio of spam vs not-spam is coming from peer instances and block any that exceed a certain ratio. It could easily be made automatic with manual intervention for edge cases.
One issue I could see is that it could be used as a way of blacklisting smaller instances from larger instances by using bot accounts on the larger instances to mark the smaller instance's legitimate traffic as spam. It would likely be necessary to implement a limit on how young/active an account can be to mark comments/posts as spam, as well as rate-limiting for situations where a given smaller community that is a subset of the larger one decides to dogpile on a smaller instance in an attempt to block them from the entire community.