I think I have an idea of what the bittersweet ending to Game of Thrones will be. The White Walkers are obviously going to exterminate a great many people once they breach the wall. I think by the time the new Long Night is over about two thirds of Westeros’s population will be wiped out, a large number of noble houses will be gone, and most of the main characters will be dead. If Jon Snow is the Prince That Was Promised, I think he will be one of the few main characters to survive, but he will be the last person with either Stark or Targaryen blood. In the end, Jon would have to lead the survivors to rebuild their way of life, but he will do it all alone. Therefore, a new undisputed monarchy would rise from the ashes with Jon Snow as its founder. In a sad irony, he would reclaim his birthright as the true king of Westeros, but he will lose his entire family to the Long Night.