So during tonight’s episode during Bran’s training he interacted with the past. Then the three eyed Raven revealed he has been in the tree for a thousand years. Right before he offered that timeline information he said that if you stay too long in the past you can not return. A thousand years before this, the historical figure Bran “The Builder” defeated the white walkers and built the wall. . .
So my theory is that Bran gets trapped a thousand years in the past. He helps defeat the white walkers builds the wall and becomes the Three Eyed Raven and waited a thousand years to train himself to save his family.