From EW: Game of Thrones actor says finale twist was George R.R. Martin's idea
“[Creators] David [Benioff] and Dan [Weiss] told me there were two things [author] George R.R. Martin had planned for Bran, and that was the Hodor revelation, and that he would be king,” Wright said. “So that’s pretty special to be directly involved in something that is part of George’s vision. It was a really nice way to wrap it up.”
So there you have it: King Bran is apparently from Martin himself."
More at the link above..........