One possible plotline could be that the troll in the bathroom attacks Harry more brutally and he succumbs to his injuries. Another could be that Quirrell, being a servant of Voldemort even earlier, manages to directly attack Harry and kill him before he has a chance to figure out what's going on. Maybe there's a dark wizard who infiltrates Hogwarts on the very first day and targets Harry specifically, seeing him as a threat even at that young age.
Another storyline might involve a betrayal within the wizarding world. Suppose someone close to Harry, perhaps a fellow student who has been secretly turned to the dark side, sets a trap for Harry. Voldemort then takes advantage of this situation and kills Harry, which would have a huge impact on the entire wizarding community as they lose their hope symbol.
One possible plotline could be that Harry is wrongly accused and sent to Azkaban. While there, the Dementors constantly torture him with their presence, draining his happiness and hope. Another could be that a dark wizard within Azkaban has a vendetta against Harry and uses dark magic to physically and mentally torture him. Or perhaps the prison system itself, with its harsh conditions and lack of proper care, is a form of torture for Harry, and he has to find a way to endure and escape while dealing with the psychological trauma.
A plotline might involve the magical community's reaction. Since Grindelwald was a feared dark wizard, when the news spreads that Harry is his grandson, there could be a lot of chaos. Some might fear that Harry has inherited Grindelwald's dark tendencies. However, Harry could use this situation to prove that one's upbringing and choices matter more than blood relation. He could embark on a journey to clear the name of the Potter family and show that he is different from his grandfather.
One possible plotline could be that Harry uses his magic in self - defense and this leads to a whole new set of challenges as he tries to hide it from the wizarding world while dealing with Vernon's continued abuse. Another could be that the Dursleys finally get caught by the wizarding authorities for their mistreatment of Harry, especially Vernon's torture.
Perhaps Harry is held captive in a hidden location by Voldemort. While being tortured, he has visions of his past and the sacrifices his parents made. This gives him the strength to resist Voldemort's attempts to break his will. His friends then embark on a dangerous quest to find him based on these visions Harry manages to send out magically.
A possible plotline is that Naruto is killed by a powerful curse placed on him as a child. Maybe it was a curse from a long - forgotten enemy of the Uzumaki clan. His death would cause a chain reaction. Hinata might be so heartbroken that she devotes her life to finding a way to bring him back. Sasuke, on the other hand, could go on a dark path of revenge, believing that Konoha didn't do enough to protect Naruto. And Kakashi would be filled with guilt for not being able to safeguard his student.
Well, a plotline might involve Harry being hunted by certain dark forces who believe that as the reincarnation of Morgana, he has the key to unlocking some great, dark magic. At the same time, he could be trying to find a way to use his connection to Morgana in a positive way, perhaps to defeat a new, greater evil that Voldemort was just a precursor to. There could also be a sub - plot where some of his friends initially doubt him but then come to realize that he is still the same Harry they know and love, just with a different heritage.
A plotline could be that Harry is on a journey to find out the truth about his parentage. He might search for hidden memories or secrets that were buried long ago. Along the way, he could encounter new magical creatures or dark forces that are interested in his unique situation. Maybe he has to go to places that were once forbidden, like certain parts of the Forbidden Forest or ancient tombs. And as he discovers more about his connection to Voldemort, he also has to deal with the fact that Dumbledore might have known something all along and kept it from him.
Quirrell could be the main villain. He was already under Voldemort's influence in the original story. It's possible that in this fanfiction, he doesn't hold back and attacks Harry more directly and lethally during his first year. Maybe he uses his position as a teacher to get close to Harry and then strikes when he has the chance.
One possible plotline could be that Harry discovers his celestial nature and has to learn to control his newfound powers. He might face challenges from both the wizarding world, who are suspicious of his new abilities, and the celestial realm, where there could be those who oppose his presence.