We have a concept of OWNERS files. Depending on which directories are getting modified, the number of directories modified, and the config settings of the directory/repo we'd like to automatically some people as reviewers. This can't be easily expressed in Herald (and hurts usability since arc would prompt for "no reviewers"). Ideally we'd be able to write some custom Python code that returns a list of users to add to the 'reviewers' or 'subscribers' line.
In a somewhat more generic sense it would be nice if we can construct the entire template: some teams want special information in the summary section or have a special format for the title section.