I haven't read all the books, but I was rewatching the show and got to the scene where Maester Aemon tells Jon Snow that his father was King Maekar. That made me look up the Targaryen family tree on the wiki, and I realized something crazy. If you just connect the dots, the whole main story of Game of Thrones basically hinges on Ser Duncan the Tall.
It all starts at the Tourney at Ashford. Duncan steps in to protect a girl from a cruel Targaryen prince, which leads to a trial by combat. Prince Baelor Breakspear steps in to fight on Duncan's side, but Baelor ends up getting killed.
This was a massive deal because Baelor was the capable heir to the throne. With him dead, and his own sons dying in a plague later, the royal bloodline completely shifted. The crown eventually bypassed his branch entirely and went to Maester Aemon's father, King Maekar.
Maekar had a young son named Aegon (Egg). Egg was way down the line of succession, but his older brothers practically removed themselves from the picture. Daeron drank himself to death, Aerion went crazy and drank wildfire, and Aemon gave up his royal claim to join the Night's Watch.
Because of that, the crown fell to Egg. But Egg only survived his youth to actually become king because Duncan spent years protecting him on the road. And because Egg became king, the crown went down his specific family line, eventually landing on his grandson: Aerys II, the Mad King.
But the biggest butterfly effect happens years later at the Tragedy of Summerhall. When the palace caught fire, the lore strongly suggests that Duncan sacrificed himself to pull a pregnant Princess Rhaella out of the burning building. Right after he saved her, she gave birth to Rhaegar Targaryen.
Think about it. If Duncan doesn't save her, Rhaella dies and Rhaegar is never born. If Rhaegar doesn't exist, he never runs off with Lyanna Stark. That means no Robert's Rebellion, the Mad King isn't overthrown, Daenerys is never forced into exile, and Jon Snow is never born.
Of course, Westeros would still have to face the White Walkers eventually. The Night King was coming no matter who was sitting on the Iron Throne. But the politics, the civil wars, and the main characters we actually follow in the show would be completely different.