There are certain projects (and users) which should never be a reviewer on a diff. For example, we have a project that consists of "all committers" which is useful for policy and other reasons. However, adding 300+ people to the emails and having them show up as a reviewer is something we want to avoid.
It would be nice if we could set a Herald rule "if any of X Y OR Z are reviewers" then "remove X Y and Z as reviewers".