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.
In some 'harry potter dies fanfic', it might be due to a failed attempt to save his friends. For example, he could be trying to protect Hermione and Ron from a deadly curse, but misjudges the situation and ends up taking the full brunt of it himself. This would cause a great deal of grief and a shift in the dynamic among the remaining characters.
One possible plotline could be that Harry sacrifices himself to defeat a new and powerful dark wizard. In this scenario, his friends are left to pick up the pieces and continue the fight against the remaining dark forces. Another plotline might involve Harry being poisoned by a mysterious and undetectable substance, leading to his slow and tragic death while his friends search desperately for a cure.
A plotline could be that Ron is killed in a magical accident during an experiment gone wrong. This accident might be related to some new, dangerous magic that he was trying to master. His death would shatter the trio. Harry would feel guilty because he might have been involved in the experiment in some way. Hermione would be grief - stricken and would pour herself into research to try and understand what went wrong. The fanfic could then explore how the wizarding community reacts to Ron's death, with some blaming Harry and Hermione, and others mourning the loss of a great wizard. It could also show how Harry and Hermione have to grow and become stronger individually to face the challenges that come their way after Ron's death.
Another idea is that there is a powerful dark wizard other than Voldemort at Hogwarts. This wizard has been hiding in plain sight all along. In Harry's fourth year, he decides to make his move. He uses a powerful dark spell during one of the Triwizard events. Harry, being in the wrong place at the wrong time, is directly affected by the spell and dies instantly, without anyone really knowing who was behind it at first.
A plotline might involve Harry being reincarnated as Smaug in a parallel universe. As Smaug, he remembers his past life as Harry and tries to use his knowledge of magic to rule over the lands in a more benevolent way than the original Smaug. Or perhaps there is a prophecy that states only a wizard - turned - dragon can save both the wizarding world and Middle - earth from a great evil, and Harry is the chosen one to become Smaug.
One possible plotline could be that it explores the emotional aftermath for Sakura. Maybe she has to deal with feelings of loneliness and guilt, even if she wasn't directly responsible for their deaths. It could also show how she rebuilds her life without her family's support.
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.
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.
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.
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.
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.