Ever since the first time Bran Stark’s warging skills were introduced in Game of Thrones, fans have speculated about the extent of his powers. When we first see Bran travel to the past, it’s as if he’s only an apparition—not really there, just watching. We now know that Bran has more power over changing past events than we thought possible. He isn’t just a spectator, he is a willing participant, as we saw in the scenes where he called out to a younger Ned Stark and caught his attention, as well as when he unknowingly messed up Hodor’s mind.
The mysterious magic that takes Bran to the past lets him change it, which is why fans have started to wonder whether Bran might be responsible for other key events that took place in the past—namely, making King Aerys II Targaryen, a.k.a. The Mad King, go insane. One fan believes that a well-intentioned act on Bran’s part could have inadvertently led to the wars we see happening in the present on Game of Thrones.
Redditor mvmchugh believes Bran went back in time to warn the Mad King (before he was the "Mad King") about the White Walkers, but was actually the cause of his madness after he told Aerys to “burn them all,” referring to the undead. Just like when young Hodor heard “hold the door,” Bran's warning messed with Aerys's mind—resulting in him going insane and wanting to burn the whole city down.
The theorist brings up the fact that in the season 8 premiere, Bran calls Jaime Lannister an “old friend.” While fans thought he might’ve meant this sarcastically, as Jaime is the reason he’s paralyzed, the Redditor thinks he called him that because Jaime is the only one who ultimately stopped the Mad King (which is how he earned his "Kingslayer" title. If it was Bran’s fault that the king went mad in the first place, he would owe Jaime thanks for saving King’s Landing.
The theory may seem a bit out there, but if there's any truth to it, we have been seriously underestimating the full power Bran has on the outcome of both the future and the past. With a popular fan theory arguing that Bran is actually the Night King, it’s clear we’re expecting something major to come out of the youngest living Stark's abilities in the final season. All we can do is watch it to find out.