Crackpot theory time: The stone giant that had no head, but darkness and black blood from the vision Bran has is not Ser Gregor, but Littlefinger. The other two from the vision are clearly Jaime and The Hound, but they are standing over Arya, Sansa, and Ned. The Hound and Jaime had a lot to do with these respected characters stories, but I really see no reason why Ser Gregor would be involved. The closest he's gotten to one if the Starks was when he captured Arya.
Littlefinger towers over them all, and is the biggest current threat to House Stark. The darkness represents his dark heart and soul, his headlessness is a reference for his need to consume everything, and the black blood represents his low birth.
Edit: No, on second thought, the beautiful man armored in gold is Joffrey. He was a direct threat to all 3 of them, but Littlefinger still towers over him.
This post was edited on 11/19 at 5:40 pm