A possible plotline could be Harry being sent to a different school as part of an exchange program. At the new school, he has to prove himself all over again. He meets new friends and enemies. For instance, if it's Beauxbatons, he might be challenged by the students who think he's just a famous name from Hogwarts and not really that talented.
One possible plotline could be that a character from Harry Potter accidentally ends up in the world of High School DxD. For example, Ron could be transported there and have to figure out how to get back while dealing with the devils and angels in that world.
One possible plotline could be that a new strain of magical typhus breaks out in Hogwarts. The students and teachers have to figure out how to contain it while also dealing with the regular Hogwarts drama. Maybe Hermione tries to research a cure using both magic and Muggle medicine knowledge.
In a 'poison Harry Potter fanfic', a plotline might involve a poisoning attempt on a major character like Dumbledore. This could send the whole wizarding world into chaos as they try to figure out who the culprit is. Harry, Hermione, and Ron would then be on a quest to find out the truth. They might suspect Snape at first, but as they dig deeper, they discover a much more complex conspiracy involving other dark wizards. Another possible plotline could be that a magical poison is used to control the minds of some wizards, and Harry has to break the spell and find the antidote to save his friends and the wizarding world from being taken over by the poison - controlled minions.
A plotline could involve a Zorua being brought to Hogwarts by a new transfer student from a far - off land. This Zorua has special powers that are not fully understood in the wizarding world of Harry Potter. The student is initially shunned because of the strange creature, but Harry and his friends see the potential good in it. As they get to know the Zorua, they discover that it can help them in their fight against Voldemort. Maybe it can create illusions to mislead Death Eaters or use its shape - shifting abilities to infiltrate their ranks.
One possible plotline could be that Harry is cursed and slowly turns into a dragon. As he changes, he has to deal with the physical and mental challenges of this transformation. His friends would try to find a way to reverse the curse while Harry struggles to control his new instincts.
One possible plotline could be Naruto trying to find a way to visit the Harry Potter world. He might search for some kind of magical portal or a powerful spell that can transport him there. Another plotline could be Naruto using the ideas from Harry Potter books to improve his ninja techniques. For example, he could learn about the concept of wandless magic and try to apply it to his chakra control.
A third plotline could be that Harry's choice of electives attracts the attention of a new and mysterious teacher. This teacher has their own agenda and uses Harry's interest in the electives to manipulate him into doing things that may or may not be in his best interest. It could lead to a lot of self - discovery for Harry as he tries to figure out the teacher's true motives while also enjoying his electives.
One possible plotline could be Harry coming to power after a great war in the wizarding world. He then has to deal with rebuilding and unifying the different wizarding communities under his rule as emperor. He might face resistance from some traditional wizarding families who don't want to accept a new form of government.
One possible plotline could be that Harry Potter sacrifices himself to defeat an even more powerful dark wizard than Voldemort. Maybe this new enemy has a power that can only be countered by Harry's self - sacrifice, which would lead to a very bittersweet ending for the wizarding world.
Perhaps a dark wizard has cast a spell that affects the hearts of several Hogwarts students. A group of healers and students with knowledge of both magic and medicine work together to develop a unique form of heart surgery to counter the spell. This could involve researching ancient magical texts and modern muggle medical techniques. The plot could be full of suspense as they race against time to save the affected students.