This is probably the shortest theory I have written, but it seems to have a powerful impact on the Game of Thrones world.
Getting straight to the point, from season 1 we have seen Bran playing the victim because of his crippling injury.
Following his escape from Theon's clutches, Bran journeys beyond The Wall to meet the Three-Eyed Raven.
Here is where things get interesting. After a meeting with the Children of The Forest and the Three-Eyed Raven, this is what Bran was told:
You will never walk again, but you will fly
At first we may think he's referring to his warg abilities, since he'll warg into birds and such.
But what if this is referring to flying beyond just warging into an animal? it means he'll fly as Bran himself, and not by warging?
How would he fly though? Hard to believe he'll grow wings. Maybe by riding a dragon, but Dany's are pretty far away. Maybe there'll be more dragons (ice dragons?)
We also have prophecy that states the Dragon has three heads. We pretty much know at this point that Dany is one of the heads and Drogon is her ride. So we have Visierion and Rhageal up for grabs, so could one of them be a ride for Bran if my theory is right?