Looking at available options under "Email Preferences > Maniphest Tasks" on https://phabricator.wikimedia.org/settings/panel/emailpreferences/ (docs: https://www.mediawiki.org/wiki/Phabricator/Help/Managing_mail ) and looking at popular personal Herald rules, it seems that

• "Notify me when I get explicitly mentioned"
• "Notify me when I become a task's owner"

would be useful additions (as their lack made some of our userbase set up expensive Herald rules as a workaround).