Almost a year ago, George R.R. Martin revealed in an Entertainment Weekly interview that he was contemplating a twist to the story of a character—and a group of characters connected with them—that he had never contemplated before and which, more notably, was something HBO’s Game of Thrones could not do because of choices the show had made that precluded it:
In one intriguing new wrinkle, Martin says he just came up with a big, revealing twist on a long-time character that he never previously considered. “This is going to drive your readers crazy,” he teases, “but I love it. I’m still weighing whether to go that direction or not. It’s a great twist… But it’s nothing I’ve ever thought of before. And it’s nothing they can do in the show, because the show has already—on this particular character—made a couple decisions that will preclude it, where in my case I have not made those decisions.”
The fandom was abuzz with it—it sparked several threads on our forum, for one—as people puzzled over who it might be. As I recall, at the time speculation was centering on Sansa Stark because of signs that the show was going to diverge heavily from her storyline in A Song of Ice and Fire (as it indeed did). However, a brand new interview with George from IGN revisits the topic and reveals some significant new information:
The two key points that George added: 1) Yes, where last year it was an idea he had had, now he’s going ahead with it; 2) It involves a character who lives in the books but is dead in the show, explaining what he meant by the show’s choices precluding it.
Speculation is once again running rampant, especially as this last season added some notable characters to the list of the dead who remain alive in the novels and who could be considered "long-time" characters: Myrcella Baratheon, Stannis Baratheon, Selyse Baratheon, Shireen Baratheon, Barristan Selmy, and Mance Rayder. Of course, other characters are possible, given that the show has killed off a few in earlier seasons that continue now, but it feels quite likely that George’s plans has to do with one of the above characters.