Bran is also the Night King
This one is predicated on a whole lot of belief suspension and time-traveling shenanigans. So this theory says that at some point in adult Brans life he uses his Three-Eyed Raven abilities to go back in time to warn the Children of the Forest of the dangers to come and try to stop them from creating the Night King. Only when he travels back to warn them, he sounds absolutely crazy because time-travel sounds ludicrous! The Children then capture a paralyzed and grown Bran and decide to sacrifice him to the Old Gods and shove a piece of dragon glass into his heart and inadvertently turn him into the Night King. This would explain why the Night King is so dang powerful, able to sense and touch Bran from the past who was green seeing into the further past which we saw back in season 6. Also when you watch the scene where Bran is marked by the Night King, there are some stark similarities between the man being sacrificed and Bran, including the position in which they are on the ground, Bran’s reaction to the man being stabbed, Bran flinches. So Bran tried to stop the Night King but then becomes to Night King… does you head hurt yet?