I recently worked with a partner who had a server with an offline agent they were unaware of. Currently, the only notification method for offline agents is if the Unresponsive Agent Settings are configured to Uninstall after X amount of days. When this occurs, an email will be sent to notify the partner of the uninstallation, and, of course, this will alert them that something is wrong if it's unexpected. They suggested adding an option for "Alert via Email/SMS" to the Unresponsive Action category, and I think this is a great idea.