Game of Thrones is right around the corner and the number of theories that are circulating on the net is staggering. Fans from various part of the world all are believing in the same theory that Bran Stark is the Night King. Although there have been many theories that have been countering that point but believers of that theory have put forward some pretty strong points.
Ever since the last episode ended with one of Daenerys Targaryen's dragon Viserion becoming the Night King's dragon, fans have gone berserk. This means that the white walkers have become more powerful and that only something very powerful can stop them. And till now, it's unclear if it will be Dragon or Rhaegal who can defeat them. Perhaps they might even if they team up to take on the white walkers.
If they fail then the only alternative is someone who will be on the living side and who can take control over others via the mind and that someone is no one other than Bran Stark. It has already been predicted by Blod Raven that Bran will fly. This, coupled with the fact that the book says "The dragon has three heads", has increased the chances of those three being Daenerys Targaryen, Bran and Night King is high.
Also, in a post on Reddit, a user has picked out evidence that points out that Rhaegal also can be one of the dragons that will defeat Viserion. Because Viserion has been described as having mossy green scales, a colour associated with the Children of The Forest and Bran's other name, Greenseing, was given to him for his gift of looking into the past and the future is.
Apart from that, there have been many previous instances where the Night King had spared Brandon Stark because the Night King's ability to see the past and the future told him that he has a big role to play in the upcoming season. And that has been proven true in the past also when Bran gave him access to The Three-Eyed Raven.
Well, now it all depends upon the next few days until the last season starts.