Emiko Queen might have stopped her brother, Oliver, from becoming the Green Arrow. Arrow season 7 episode 17, "Inheritance," flipped the show's storylines sideways, revealing that the new Green Arrow was the true villain of the story - and has been for some time.
"Inheritance" explained the connection between Emiko and Dante through a series of flashbacks, which showed how a tween-age Emiko came to be recruited by Dante after she tried to pickpocket his watch and how he trained her in the arts of combat. These scenes also gave a name to Dante's organization, The Ninth Circle, which confirmed they were meant to be an adaptation of a recent addition to Green Arrow's rogues gallery in the comics. Finally, these flashbacks showed how Emiko came to fully embrace the Ninth Circle and secretly became a part of its leadership, with Dante acting as both her second in command and the public face.
At one time, Emiko Queen had been interested in trying to reconcile with her estranged father, Robert Queen, and proving her worth to him as a business asset. Dante learned of this and offered Emiko an envelope, which he said contained a gesture of the Ninth Circle's respect for her and that he would leave it to her to decide where her loyalties lay. The final scene of "Inheritance" revealed the envelope's contents - blueprints for Robert Queen's private yacht, the Queen's Gambit, and proof that his business associate Malcolm Merlyn was planning to plant a bomb on the yacht. Emiko burned the plans as she watched the news report of the Queen's Gambit's apparent destruction, having decided against giving them to her father after he once again rejected her.
As every devout Arrow-head knows, the bombing of the Queen's Gambit would have far-reaching consequences for both Oliver Queen and the Arrowverse as a whole. Had it not been for Malcolm Merlyn's plans to kill Robert Queen, Oliver Queen would never have gone on to spend the "five years in hell" which gave him the skills he needed to become the Green Arrow. The bombing also started Sara Lance's journey to becoming a member of the League of Assassins and her trying to find redemption for her past life - first as the vigilante Canary and then as one of the Legends of Tomorrow. And without Green Arrow to mentor Barry Allen in the days before he became The Flash or White Canary captaining the Waverider, the current state of the Arrowverse would be radically different.
None of this is to say Emiko's decision is a pivot point for the whole timeline. Barry would have still got his powers and this doesn't account for other Earths either. But so much of what defines the narrative of The CW's interconnecting superhero shows is now rooted in her twist decision.
While a retcon, there is certainly a great deal of irony to Emiko Queen's actions. By acting selfishly and allowing her father to go on to his death, she was indirectly responsible for starting two people down a path of pain that would ultimately lead to a far greater good than had she remained loyal to her father or tried to avert a tragedy herself. This builds upon the further irony that Malcolm Merlyn would ultimately create the same heroes who would go on to thwart his plans on multiple occasions. Emiko's twist in Arrow may be shocking, but goes to highlight the power of unintended consequences and how small acts can build great things.