Idea for a more capable relay server

Very often I hear from users and ex-users of the Fediverse, that they they complain about posts that never reached their accounts. For example they subscribed to a hashtag on two accounts on separate servers. Now they see that both servers deliver different posts.

A solution for that are relay servers. But not every admin wants to receive thousands of unwanted posts just receive the few ones with content that a user wanted.

A solution for that would be a relay server that would be able to deliver content based on some rules that the admin defined. This should be much more than just a list of hashtags. It should be possible to filter on content in any field of a message.

What do you think about this idea? I’m currently in the process of thinking about a way to communicate these filter conditions.