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.
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.
Physical therapy can also be helpful in some cases. Exercises to strengthen the pelvic floor muscles can improve bladder control. Kegel exercises, for instance, are often recommended for women with bladder problems. They can be done regularly at home and may gradually reduce the reliance on diapers.
One interesting scenario could be when Harry is forced to wear a dress during a magical prank gone wrong at Hogwarts. His friends' initial reactions of shock and then laughter can add humor to the story.
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.
Medical reasons are common. For example, if she has a bladder or bowel disorder, diapers can provide a practical solution to manage incontinence. It helps her to go about her daily life with more ease.
One possible scenario could be Naruto being constantly ostracized in the village. People might avoid him not just because of the Nine - Tails inside him, but also due to false rumors spread about him. For example, they could say he's a bad omen. This would lead to Naruto feeling extremely lonely and isolated, and he might start to train alone more often, distancing himself further from the possibility of making friends.
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.