The Age of Sigmar facebook page has confirmed in a reply to a post that all Battletomes will be getting content in the form of Siege Warfare Alliegence Abilities in the upcoming Wrath of the Everchosen book! And better still, this includes the Seraphon!

We are looking forward to seeing some Age of Sigmar siege Warfare, seems a good time to bring the old citadel fortress out of retirement!