WARNING: This list contains spoilers for the entirety of Game of Thrones, including book spoilers. Voting on theories will be closed when they are either disproven or come to fruition.
It's clear by now that Bran Stark has a major role to play in the finale of Game of Thrones, but what exactly that role is remains a mystery. As the Three-Eyed Raven, Bran is in a unique position to influence the outcome of the war against winter. His Greenseer abilities allow him to view and even alter the past and future, and they grant him the skill to see events happening hundreds of miles away. On top of that, he is a master warg and the only person we've seen with the power to warg into another human being. With his near omnipotence and ability to control other living beings, he may just be humanity's last hope to beat back the Long Night.
Game of Thrones fans have cooked up a million different theories trying to puzzle out exactly how the series is going to end, and many of them revolve around Bran. Is he the one who will finally slay the Night King, or will he unintentionally become the thing he was supposed to fight against? It's possible that we will see Bran fly, time travel, become a White Walker, and die all in one season. We will all have to wait until April 14th to get our final clues into the Game of Thrones endgame, but these theories might give us some early hints of what is to come.
Voting Closed: Samwell Tarly told Jon Snow about his true heritage in Season 8, episode 1 ("Winterfell")
Season 7 saw one of the most popular Game of Thrones fan theories, R+L=J, come true. Jon Snow's parents were Rhaegar Targaryen and Lyanna Stark, making him the rightful heir to the Iron Throne.
Together with Samwell Tarly, Bran managed to put together the whole story and reveal Jon's true identity. Jon still doesn't know about this, however, and it's likely that his younger brother (or cousin, technically) will be the one to break the news to him.
Redditor /u/mjgffect7 goes as far as to say that Bran will reveal Jon's identity by connecting him to Ned's spirit via Weirwood. Of course, Sam Tarly also knows the truth about Jon. It could be him that breaks the good news instead of Bran, so we'll have to wait and see how this one plays out.
Bran's powers of greenseeing and warging give him more power to manipulate the events of Game of Thrones than any other character, save maybe the Night King himself. He has already affected the past on two occasions, whispering to his father at the Tower of Joy and warging into Hodor when he was still a boy named Willis.
With the Army of the Dead on the march, the stakes have never been higher. It's highly likely that Bran will try and manipulate the past in order to stack the odds in favor of the living.
"You'll never walk again, but you will fly."
This is what the Three-Eyed Raven tells Bran the first time they meet, and it was a line that got fans instantly excited. Redditor /u/MillerTime5858 believes this bit of foreshadowing is literal, and that Bran will actually fly at some point in the future.
The most likely case for this would be Bran warging into a dragon, although he might physically ride one as well. Either way, it's a pretty solid bet that we will see this happen in some form this season, unless the Three-Eyed Raven was being purely metaphorical.
Voting Closed: While the two shared a meaningful glare, no mind battles happened before Arya slayed the Night King in Season 8, episode 3 ("The Long Night")
Redditor /u/YezenIRL put forth a theory that they are pretty confident is going to happen, even if they're not entirely happy about it.
YezenIRL theorizes that Bran and the Night King will partake in a "mindbowl," a mental battle that takes place outside of the physical world. It makes sense, as we've already seen the Night King touch and harm Bran in one of this visions.
The idea is that while Jon Snow faces the Night King on the battlefield, Bran will simultaneously attack him on the mental battlefield. While Bran distracts the Night King, Jon will be free to strike the final blow.
The theory's original author isn't the biggest fan of this route, explaining that mental battles are hard to capture in a visual medium like television. It might just be too weird to play out on screen, although nothing is impossible.