Well, it could be that Harry becomes an oni demon through some sort of dark magic accident. Once he turns, his personality might start to change slightly. He could become more aggressive or have a different view on things. His relationships with his enemies would also be different. Instead of just being a wizard fighting them, he now has the powers of an oni demon, which could make his battles more intense and unpredictable.
In this kind of fanfiction, Harry might face a lot of challenges. Maybe he has to adapt to his new oni demon form, which could include dealing with new powers like enhanced strength and strange demonic abilities. He might also struggle with how others perceive him, especially his friends who knew him before. There could be a whole new world of demon politics and hierarchies that he has to navigate.
The story could start with Harry being in a really strange situation. Maybe he was exploring a forbidden area and got cursed. As an oni demon, he might have to find a way to reverse the transformation at first. But as time goes on, he realizes that there are some advantages to this new form. He could use his oni demon powers to protect his loved ones in ways he couldn't as a normal wizard. His journey would be about finding a balance between his old self and his new demonic self.
Perhaps the traditional enemies of Harry in the wizarding world would also become more hostile. They might be scared of his new powers and try to take him down before he becomes too powerful. And there could be some powerful oni demon lords who want to control Harry and use his unique situation for their own gain.
If Harry Potter becomes a demon in fanfiction, his relationships would be severely affected. His friends like Ron and Hermione would be very confused and possibly afraid of him. They might try to find a way to reverse the transformation. And Hogwarts itself would be in chaos. The teachers would be divided on how to deal with him, some might want to help him while others might see him as a threat to the safety of the school.
In such a fanfiction, Harry might gain the ability to rise from ashes, literally and metaphorically. He could have the power of regeneration and immortality like a Phoenix. This would change his battles, for example, he could fearlessly face Voldemort as he could always come back even if defeated. His relationship with his friends might also change. They would see him as an even more extraordinary being, and he might have to deal with their awe and a bit of distance that could come from his new form.
Well, if Harry Potter becomes a villain in fanfiction, it would be a complete subversion of the original story. He might use his powerful magic skills to dominate Hogwarts instead of protecting it. He could form alliances with the dark forces that he once fought against. Maybe he would start to spread fear among the students and teachers, and turn the school into a place full of chaos and tyranny.
Well, it could be really interesting. Harry Potter, with his magical background from Hogwarts, would find the world of Witchers quite fascinating. He might be intrigued by the Witchers' potions and their unique way of fighting monsters. Maybe he'd try to combine some of his spells with the Witchers' combat skills. For example, he could use a Patronus charm while also slashing at a monster with a Witcher's sword. And he'd surely be interested in the different kinds of monsters in the Witcher world compared to the ones he faced in the wizarding world.
In such a fanfiction, Harry Potter becoming a Darth Vader fan could lead to him adopting some of Vader's more authoritarian and decisive ways. He might start to use more forceful methods to deal with his enemies in the wizarding world, which would surely cause a stir among his friends and foes alike. For example, instead of relying solely on magic spells in a traditional way, he could be influenced by Vader's use of the Force and try to develop something similar in the magical realm.
Well, it could be a very dark plot. Maybe Harry is under some dark magic influence that makes him lose his mind and start killing. It would completely change his character from the original story.
If Harry Potter becomes an obscurial in fanfiction, the story could take a very different turn. He might not be as eager to attend Hogwarts as he was originally. The Dursleys might have an even worse time with him because his obscurial powers could lash out. At Hogwarts, teachers would be extremely concerned. Snape, for instance, might be even more critical of him, seeing him as a potential danger. And Harry would have to learn to control this new and dangerous aspect of himself.
If Tanjiro becomes a demon in fanfiction, he might lose his sense of humanity. His relationships with his friends and family would be severely strained. For example, his sister Nezuko, who is also a demon, might be affected in unexpected ways. He could struggle with his newfound bloodlust and the need to consume humans, which goes against his original kind - hearted nature.
In this fanfiction, the plot might show Harry's transformation into a Dark Lord as a result of some major betrayal in the wizarding world. For example, if the Ministry of Magic turned against him or if his friends were seriously harmed by the 'good' side. He could then start to see the power of the dark side as a means to exact revenge and reshape the wizarding world according to his own vision. He might recruit former Death Eaters or create his own dark followers.