One interesting plot twist could be that Harry discovers he has a long - lost sister who is also a powerful witch. This would completely change the dynamics of his family and his place in the wizarding world.
In some 'Harry Potter Twist of Fate' fanfictions, a major plot twist is that Voldemort was actually being controlled by an even more powerful dark wizard all along. This would rewrite the whole conflict in the series and make Harry's fight against the true mastermind much more complex. For example, this could lead to new alliances being formed as the wizarding world realizes they were fighting the wrong enemy for a long time.
Chapter 727, "Can't Keep Up with the Twists Anymore", is the clearest early answer because it turns Jinli Studio's situation into a concrete plot event. It opens with Jiang Li only posted one picture, but it shut up those haters.
Chapter 665, "Give You a Good Lesson on Behalf of Your Mother!", is the clearest early answer because it turns Sister Liu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Why should I get so many points?
Chapter 707, "I am Daoist Qingfeng", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Su Yinwan hadn't finished her dessert but, feeling agitated, had...
Chapter 837, "Why did she come?!", is the clearest early answer because it turns Gu Shengrong's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The ring is the one Shi Lei just gave her, she really likes it, so...
Chapter 528, "Cursed", is the clearest early answer because it turns Shang Shaobai's situation into a concrete plot event. The chapter gives readers a real answer by opening on The netizens almost unanimously started to sympathize with Jiang...
Chapter 563, "Quite the Confidence", is the clearest early answer because it turns Chen Manman's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Upon learning the result, the four of them stood frozen on the spot,...
Chapter 551, "Who is Reciting "Student Rules"?", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. The chapter turns that into something concrete by moving from Other competitors were already in place, and Qi Tianyu was the last...
A practical answer is Chapter 23, because it turns the idea into a concrete scene. The chapter opens with Although Jiang Li exerted very little strength, Jiang Yan instantly felt..., adds pressure through So the moment the news broke that Jiang Li was entangling with Shang..., and ends on However.
Chapter 705, "Is She Her Own Destined Noble Person?!", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with He Wen was made to laugh by Bai Lu. and follows through with So he's just a side character, huh..
Chapter 482, "Someone Might Need Me", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The chapter turns that into something concrete by moving from Other people caught a glimpse of Zhao Lang's cold and stern face and...