A decent enough solution **_would be_** hashtags, and new users are suggested hasgtags instead of npubs - would be, but isn't, because people abuse hashtags and then people unfollow the hashtags. I suggest limiting the number of hashtags that can be added to a note, or relayed as a relay policy - 3 tags seems like a good maximum. That way, interest groups break off into their own silos, instead of the current psychology of an implied demand for conformity.