Well, perhaps they are in the Forbidden Forest together for some reason. Maybe they got detention and had to go there to collect some rare ingredient. A group of rogue centaurs attacks them. Draco, who has a better understanding of the politics among magical creatures due to his family's influence, manages to talk the centaurs down and in the process, saves Harry from getting hurt. Or it could be in a Quidditch match where there is some dark magic involved. Draco realizes that Harry's life is in danger from a charm placed on a Bludger, and he uses his own flying skills and a bit of magic to deflect the Bludger and save Harry.
One possible plotline could be that in a dangerous magical situation, like a dark wizard attack during a Hogwarts event. Draco, despite his previous animosity towards Harry, has a change of heart. Maybe he overhears some information about the attack being part of a larger plot against Harry specifically. So, Draco steps in and uses his family's knowledge of certain spells or magic artifacts to save Harry. Another could be in a potion - making class gone wrong where Harry is about to be seriously injured by a rogue potion effect, and Draco, who is actually quite good at potions, quickly whips up an antidote or counter - spell to save Harry.
In some fanfictions, it could be a case of a powerful curse that forces Harry to act against his will. The curse might be placed on him by a powerful dark wizard who wants to create chaos in the wizarding world. Draco is just an unfortunate victim of this curse as Harry, unable to control himself, kills Draco. This then sets off a chain of events where Harry has to deal with the guilt and the consequences of his actions.
One possible plotline could be that they discover they are brothers early on in their Hogwarts years. This would cause a huge stir among the students and teachers. They might initially struggle to accept it, especially given their past animosity. But as they get to know each other better, they start to form a bond and protect each other from the various threats in the wizarding world.
One possible plotline could be that Harry betrays Draco in a duel. Maybe there was some sort of misunderstanding or a secret deal that Harry made with someone else that required him to turn on Draco during the duel. For example, if there was a dark wizard threatening Harry's friends and the only way to save them was to sacrifice Draco in the duel by using a forbidden spell against him.
Maybe Harry is being bullied and psychologically abused by some Slytherin students under Lucius' influence in a way that Lucius didn't initially anticipate getting so out of hand. Lucius then steps in to save Harry from this situation, perhaps to avoid a scandal that could harm his family's reputation in the wizarding community. It could also be that Harry is being abused by some dark magic experiments in the school that Lucius uncovers accidentally while looking for something else, and he decides to save Harry out of a sense of justice or self - preservation as the chaos could spread and affect his own son Draco.
In this fanfiction, a plotline could involve Harry being magically bound to Draco's mom in some way. As a result, he has to stay close to Draco to protect the bond. This could lead to many comical and tense situations between Harry and Draco as they navigate their new relationship. Another possibility is that Draco's mom has left behind a powerful artifact, and Harry is the only one who can find it. Draco, thinking Harry has ulterior motives, tries to stop him at first but then realizes they need to work together. Also, there could be a plotline where Harry accidentally uses a spell that links his mind to Draco's mom, and he has to figure out how to break the link while dealing with Draco's reactions.
One possible plotline could be that Harry has some sort of misunderstanding. Maybe he believes Draco is involved in a dark plot that endangers Hogwarts. So, in a rash moment, he kidnaps Draco to interrogate him. But as he does this, he realizes that Draco might be innocent and then has to deal with the consequences of his actions.
One possible plotline could be that Harry, through some magical accident or inheritance, comes to own Draco's family manor. Draco is then forced to interact with Harry on Harry's terms to regain his family's pride. Another could be that Harry wins a bet against Draco and the terms of the bet are such that Harry now has ownership over Draco's time, making Draco do tasks for Harry.
One possible plotline could be that Draco steals Harry's underwear as a prank. He might do it to embarrass Harry in front of their classmates at Hogwarts. Another could be that Draco steals it because he has some strange fascination with Harry and it's a way for him to have something of Harry's. Or perhaps it's part of a dare within the Slytherin group that Draco is a part of.
In a more complex plotline, there could be a misunderstanding. Harry might be trying to protect Draco from something, but Draco misinterprets his actions as Harry trying to undermine him. When Draco finally realizes the truth, he is so overcome with mixed emotions like guilt and relief that he breaks down in tears.
Maybe in a dark turn of events, Draco is under the influence of some dark magic or a curse that makes him act out against Ginny. She becomes a target because she is associated with Harry and the resistance against the Dark Arts. In this plot, Draco would struggle to break free from the control and Ginny would have to be the bigger person in the end.