In a more complex scenario, it could be part of a time - traveling mishap. Harry travels back in time and gets stuck in the body of his much younger self. Since his younger self was still in diapers, he has to deal with wearing them until he can figure out a way to reverse the time - traveling problem.
Perhaps there's a potion accident. Harry might have been experimenting with a new potion in Potions class, and something went wrong. The potion could have affected his body functions, resulting in his need to wear diapers. It could add an interesting and comical twist to the story.
It could be for comical purposes. The idea of the powerful and heroic Harry Potter having to deal with something as mundane and embarrassing as wearing diapers can be quite humorous. It adds a touch of absurdity to the usually serious Harry Potter universe.
One possible scenario could be that the magical characters find themselves on a Muggle plane due to some strange magical accident. Maybe they were trying to apparate but got misdirected into the plane. Then, when the plane starts to crash, they use their magic to try and save the day, like Harry using a spell to slow down the fall or Hermione creating some sort of magical shield to protect the passengers.
In a 'Harry Potter betrayed fanfic', perhaps Hermione turns against Harry. It could be because she disagrees with Harry's more dangerous and reckless plans to fight Voldemort. She might think he's endangering too many lives without proper thought, and this leads to a rift and a sense of betrayal.
In the 'Harry Potter Controls the Elements Fanfic', perhaps Harry discovers his element - controlling powers gradually. He first starts with something simple like controlling the wind to cool himself on a hot day at Hogwarts. Then, as he grows more powerful, he can combine elements. He could create a storm by mixing air and water elements, which he uses to protect his friends from a group of Death Eaters.
One scenario could be that he's under the Cruciatus Curse. This curse inflicts intense pain, and Harry has faced it before. In a fanfic, it could be used by a new dark wizard adversary, making Harry cry out in agony.
Perhaps in the fanfiction, Harry Potter's death occurs because he tries to rewrite a powerful magic spell that goes horribly wrong. He was always known for his bravery and willingness to take risks in the original series, so this could be an extension of that. His death would then be a tragic consequence of his own curiosity and desire to push the boundaries of magic.
One possible scenario could be that Harry is exiled due to false accusations of dark magic. Maybe the Ministry of Magic, fearing his power after the defeat of Voldemort, spreads lies about him being a threat. Another could be that he chooses exile to protect his friends from a new, unknown danger that is targeting him specifically.
I'm not sure specifically which ones are the most popular. However, fanfics in this area might explore unique scenarios within the Harry Potter universe related to diapers, like perhaps a babyfic where characters are magically reverted to an infant state and all the adventures and challenges that come with it.
One possible scenario could be that Hermione finally realizes her deep - seated feelings for Harry. In the fanfic, perhaps after a particularly dangerous adventure where they almost lost each other. She might approach Harry in a quiet moment, like in the Gryffindor common room late at night when most students are asleep. She could be a bit nervous but determined as she asks him out for a walk by the lake or a trip to Hogsmeade. This would be a huge moment for their relationship as it takes a more romantic turn.
One possible scenario could be that Harry, Hermione, and Ron continue to work on reforming the wizarding world. They might focus on improving the relationships between different magical communities, like the werewolves and the wizards. Another could be that Harry finally gets to have a normal life, teaching Defense Against the Dark Arts at Hogwarts and sharing his real - life experiences with the students.