In the fanfiction, the bad seizure might be related to a connection with Voldemort. Since they have a unique link, Voldemort could be using some dark magic to attack Harry's mind and body, causing the seizure. This could be part of Voldemort's new strategy to weaken Harry before their final confrontation.
Perhaps in this fanfiction, the bad seizure is a result of some internal struggle within Harry. He could be dealing with the immense pressure of his destiny, the weight of the prophecy, and all the losses he has endured, which manifests as a seizure.
It could be a new dark wizard or witch who has a vendetta against Harry. This person might have developed a new form of magic specifically to harm Harry, and the seizure is the first sign of this new threat.
In this fanfiction, the seizure could be part of a curse that was meant for Sirius but backfired on Harry. Harry starts seizing in front of Sirius, and Sirius is filled with guilt and determination to reverse the curse. He uses his knowledge of the wizarding world and his connections to search for a cure, all while taking care of Harry and keeping him safe from any potential threats.
One reason could be that Harry is under an extreme amount of stress. He has so many burdens on his shoulders in the wizarding world, like fighting Voldemort. This stress could manifest physically as a seizure, especially if he is in a tense situation in front of Snape, who is often a source of anxiety for him. For example, if Snape is about to accuse him of something he didn't do while he is already worried about other threats to the wizarding world.
It could be about how Edward reacts when Bella has a seizure. Maybe he uses his vampire abilities to help her, like getting her to a safe place quickly.
A plot could involve a symbiote attaching itself to Harry. This symbiote might have its own motives. It could be from another dimension where magic and symbiotes are intertwined. The symbiote could start influencing Harry's actions, making him more aggressive in his fight against Voldemort. But at the same time, his friends are suspicious of the changes in him. As the story progresses, Harry has to learn to control the symbiote and use its power for good while also dealing with the internal struggle of not losing himself to it.
Maybe Harry is at a Quidditch match. He's flying high in the air on his broomstick, but suddenly he feels the urge to pee. He can't just fly down to the loo in the middle of the game. His teammates notice his distracted state and wonder what's wrong with him. It could lead to some interesting interactions among the Gryffindor team members as they try to figure out what's up with Harry while still playing the game.
Perhaps the plot could be that the child of Harry Potter gets sent back in time accidentally. There, they meet a young Harry and his friends. The child has to be very careful not to disrupt the past too much while also trying to find a way back to their own time. This would be filled with lots of tricky situations, like when they almost reveal future events or when they have to avoid being seen by their own young parents.
In a Harry Potter has a son fanfiction, the plot could involve the son inheriting his father's magical abilities and facing new threats at Hogwarts. Maybe he discovers a hidden chamber that holds a powerful and dangerous magic. He might also have to deal with the pressure of being Harry Potter's son, with everyone having high expectations of him.
The plot could involve Harry finding the scythe in a hidden chamber at Hogwarts. Maybe it was left there by an ancient wizard, and the scythe has the power to cut through dark magic. Harry then has to learn how to use it to fight Voldemort in a new and unexpected way.
In a fanfiction plot, Harry could be born with telekinesis but it was suppressed by the magic that protected him as a baby. When he enters Hogwarts, the magic of the school starts to awaken this power within him. His friends are both amazed and a bit scared at first. As he hones his telekinesis, he uses it to uncover secrets in the castle, like hidden passages that were sealed off long ago. But this also attracts the attention of dark forces, and he has to use his power to defend Hogwarts and the wizarding world from a new threat that is related to an old, forgotten magic.