What if everyone dies and the Age of The Iron Throne is over? This fight is not just bringing kingdoms together, but the people themselves and after the fight the survivors realize they no longer want to be subjects and be ruled over and used as pawns by kings or queens. After such an encompassing and costly battle(s), many rulers dead and what few may remain, they no longer have the resources or power to command the remainders of humanity. Someone new or unexpected rises as a leader, a new type of leader, with a new and different structure that their world has not known before and break the feudal type system.
Way outside the box, but considering how global and catastrophic the outcome could become, it could be the type of event to completely change how things have ever been before.