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 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.
A possible plotline could be that Draco, out of curiosity, starts to research Harry Potter's mother. As he delves deeper, he finds out about her selfless love and her powerful magic. This makes him question his own values and beliefs, leading to a transformation in his character.
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 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.
The Malfoy family would surely be enraged. Lucius Malfoy would use all his influence and power to get Draco back. This could lead to a conflict between the Malfoy family and Harry's supporters, causing a rift in the wizarding community.
One plotline could be Naruto being a transfer student at Hogwarts. He might struggle to adapt to the magic system while also showing off his ninja skills. Another could be Naruto teaming up with Harry Potter to fight a common enemy that threatens both the ninja world and the wizarding world.
One possible plotline could be Harry using his knowledge of the wizarding laws, which he learned through his years at Hogwarts and his experiences with the Ministry of Magic, to defend wrongly accused wizards in a wizarding court. Another could be him trying to reform the wizarding legal system which he might find full of loopholes and biases.
One possible plotline could be Harry trying to hide his merman identity while at Hogwarts. He might face challenges in the magical underwater areas of the school, like the lake. Another could be that he discovers his merman heritage and has to go on a journey to learn more about it and his new powers.
One possible plotline could be Harry being secretly trained by an unknown figure to become an assassin against the dark forces. He would have to hide this new identity from his friends at Hogwarts.
One possible plotline could be about the Weasley family trying to improve their financial situation. Maybe they come up with a new business idea in the wizarding world, like a magical product that becomes a huge success and changes their economic status. Another could be a story about how the Ministry of Magic regulates the financial system in the wizarding world, and there are some characters who try to manipulate or reform it.