The important themes of Mockingjay may be used as you create your slogan. By this book in the series, Panem is at war, and much has changed. Panem is not the same place it was at the beginning of the series. The idea of change (and sometimes very rapid change) is a rather important theme. Many of the characters have struggled with both personal and political decisions, and have been changed by the events that they have witnessed and been a part of. Peeta has been mentally scarred by the violence and torture he has been subject to, and Katniss has only a tenuous hold on herself, as seen by her terrible nightmares and her distrust of many around her. She must make some hard moral decisions, such as whether or not to continue to hold a Hunger Games with the Capitol's children. Propaganda and publicity are also a large part of the rebellion. Throughout the series, Katniss has become known as "the girl on fire," and by the third book, Mockingjay, she is a well known figure of the rebellion. The idea of destroying the old with fire and starting anew, making change, and rebelling against old (and new) systems of authority are all ideas that could be part of a slogan for this book. The link below will take you to the eNotes Essentials for the book, which will provide you with an in depth analysis of the important themes.