We can already choose which relays we write and read from. That being said it should be automated, by relay operators only broadcasting notes that pertain to their niche. Right?!
Discussion
I mean each individual note. You could select which relay you want to post it on 
I think you then run into the siloed, not decentralized issue. If I recall a post you made while back about removing Damus relay and not being able to see a bunch of people's posts was my inspiration to build nostr:npub16fcy8ynknssdv7s487nh4p2h4vr3aun64lpfea45d7h4sts9jheqevshgh since Damus relay is one server (literally).