It's all too easy to forget that, at its heart, Game of Thrones is a story of light versus darkness. On one side of the battle is R'hllor and his champion, Azor Ahai, or the Prince That Was Promised. On the other side is a being far more mysterious than the Lord of Light — the Great Other and his army of the dead led by the Night King. As season eight approaches, there's been plenty of discussion about who Azor Ahai reborn will be, but precious few people are questioning whether or not the Great Other will be revealed to be a physical person. In the A Song of Ice and Fire books it's revealed that the Great Other's true name is never spoken, which leads to the possibility that a big reveal could be coming. Who is the god of the cold, dark, and the night? All signs currently point to the Great Other being none other than Bran Stark, and if the hints are true, then it could be the most devastating twist of them all.
As Reddit user CappieXL points out, Bran's powers as the Three-Eyed Raven are immense. He can see all of time, and having that much knowledge dumped into his head is making Bran significantly less human than he once was. Now, the previous Three Eyed-Raven appears to have been in opposition of the Night King, which makes the idea that Bran could be the god for whom the Night King is a champion a bit hard to swallow. But let's go back to the time before Bran receives all of his powers. At one point, he sees the Night King as he's on one of his trips through time, but the Night King sees him, too, and with one touch the leader of the army of the dead knows exactly where to find the Stark boy. This implies a connection between Bran and the Night King that Bran may not fully understand.
Now, the Reddit theory posits that Bran is actually R'hllor, and the Night King is the Great Other, but if we work with the origin story presented by the show, the Night King is created by the Children of the Forest. That suggests he's an agent of the Great Other, as Melisandre states — she calls the Night King and his army the "cold children" of the Great Other. Meanwhile, R'hllor is the god of light, a figure that doesn't appear to have a physical counterpart. Instead, he has a representative to do his bidding on Earth and stop the Great Other from spreading his darkness across the world. That representative is the Prince (or Princess) That Was Promised.
Just because R'hllor doesn't have a physical counterpart, that doesn't mean the Great Other doesn't either. In fact, there's evidence in the books that Bran is working with the god of the cold, even if his service is unwitting. As the A Song of Ice and Fire wiki points out, Melisandre sees a vision in the flames of "a wooden face, corpse white, a thousand red eyes, and a boy with a wolf's head beside him." The boy with the wolf's head is believed to be Bran, a theory that is backed up by the Three-Eyed Raven (known as the Bloodraven in the books) telling him: "'Never fear the darkness, Bran.' The lord's words were accompanied by a faint rustling of wood and leaf, a slight twisting of his head. 'The strongest trees are rooted in the dark places of the earth. Darkness will be your cloak, your shield, your mother's milk. Darkness will make you strong.'"
If Bran's powers are rooted in the darkness and his ability to see all things at once, then he may not have realized that he's the Great Other yet. Or perhaps he has, and he's manipulating his mortal family into position to face off against the Night King, knowing that they'll fail. Throughout season seven, it's made pretty clear that Bran as we knew him no longer exists. He's now in touch with a power that's so large that he can see everything happening at once. That kind of power could be corrupting by nature, and it could lead to someone like Jon Snow, who is strongly hinted to be Azor Ahai reborn, having to kill Bran in order to truly stop the coming darkness. Because if Bran is the Great Other, then the Night King is simply a figurehead, which means the battle facing Jon, Daenerys, Arya, and the rest of the remaining characters is far more insidious than they could ever expect.