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.
If Harry Potter becomes the Joker in fanfiction, he would probably start to question all the values he once held. The Joker is all about subverting order, so Harry might begin to undermine the Ministry of Magic and other institutions. He could use his knowledge of magic to play cruel pranks on people, not the innocent kind of pranks he used to do. His Gryffindor courage could be twisted into a reckless disregard for the safety of others.
If Harry Potter becomes a Jedi, he would likely combine his existing magical skills with Jedi abilities. His Patronus charm could be enhanced by the Force, becoming an even more powerful protective entity. He might use his wand as a sort of lightsaber hilt, channeling the Force through it for unique combat moves. His ability to apparate could be refined with Jedi - like teleportation skills. Also, his knowledge of the wizarding world's dark magic could help him better understand the dark side of the Force and avoid its temptations.
If Harry Potter becomes a Lamia in fanfiction, his relationships would change drastically. His friends might be both intrigued and a bit scared at first. He would have to adapt to his new form and its characteristics. For example, he may need to learn how to control any new instincts that come with being a Lamia. Also, his role in the fight against Voldemort could be completely different. Instead of using a wand in the traditional way, he could use his Lamia - related powers to combat the dark forces.
Well, it would be a total chaos. He might use his magic skills to cause destruction. For example, he could turn Hogwarts into a dark fortress and rule over it with an iron fist. His friends would be shocked and probably try to stop him, leading to some really intense battles.
In such a fanfiction, Harry might gain enhanced physical abilities like super strength and speed. He could also have new magical powers related to his vampiric nature. For example, he might be able to hypnotize his enemies. His relationships with his friends would be severely tested. Hermione might try to find a way to reverse his vampirism while Ron could be initially afraid of him.
In this kind of fanfiction, Harry could potentially use the Hulk's strength to fight Voldemort in a very different way. He might be able to smash through dark magic barriers with ease, and his rage - like the Hulk's - could be a new source of power against the Death Eaters.
Imagine Harry Potter getting into creepypasta fanfiction. He could begin to incorporate some of the darker themes into his own view of magic. He might explore abandoned parts of Hogwarts that he thinks could be like the settings in creepypasta stories. His adventures could take a very different turn as he looks for signs of the uncanny and the terrifying within the familiar walls of the school. His perception of good and evil could also be distorted, as the lines are often blurred in creepypasta. This could lead to some really interesting, if not a bit disturbing, character development for Harry.
Well, if Harry Potter becomes a lich in fanfiction, he would likely gain immortality. But he might also lose some of his humanity. His appearance would probably change, becoming more skeletal and undead - like. His magic could take on a darker, more necromantic tone.
In such a fanfiction, Harry Potter as a ghost might have a very different perspective on the wizarding world. He could be able to pass through walls and observe things secretly. Maybe he would try to influence the living wizards in some mysterious ways, like guiding his friends to discover hidden secrets or solve mysteries that were left unsolved when he was alive.
He might gain more independence in his actions. For example, he could make decisions about his own education and training without the interference of the Ministry or Dumbledore. He could choose to explore magic in his own way, perhaps even study some dark arts to understand his enemies better, but not necessarily use them for evil.