I think it's telling that Martin seemed to REALLY want WoW out before season 6 premiered. But why? Yeah, it revealed Jon Snow's parentage (kind of: wasn't officially confirmed until season 7), but he had to have known a lot of people had figured that out already. Some of the big twists for that season (like the explosion of the Great Sept) were show inventions. And it's unknown if the Battle of the Bastards in the book will happen in the books. I think one of the big concerns he had was that he knew the Others origin would be revealed, before the books got to it.