In a fanfiction where Harry Potter becomes the Joker, it would change the entire dynamic of the story. Harry would lose his sense of purpose that was centered around fighting Voldemort and protecting the wizarding world. Instead, he would be focused on creating anarchy. His appearance might change too, with a wilder, more disheveled look. His relationships would be shattered as he no longer adheres to the same moral code. For example, Hermione would be appalled and try to reason with him, but he would likely just laugh in her face.
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 Batman becomes Joker in fanfiction, Gotham would be in chaos. Batman is the symbol of justice and order, while Joker represents chaos and anarchy. Without Batman to protect the city, criminals would run wild. And as the new 'Joker', Batman might use his intelligence and skills in a very different and dangerous way, perhaps creating more harm than good.
If the Joker knew Harry Potter fanfiction, it would be a very odd combination. He could potentially use the knowledge of magic to enhance his already dangerous and unpredictable nature. He might take inspiration from the Death Eaters and form his own group of 'Dark Followers' in Gotham. He could also use the idea of Horcruxes and try to create something similar to ensure his immortality in his battles against Batman. However, it's also possible that he would simply mock the concepts in the fanfiction, seeing them as too 'wholesome' compared to his own brand of chaos.
In such a fanfiction, it would be a very strange and chaotic scenario. Harry Potter comes from a world of magic and the Joker from a world of dark, madcap crime. If the Joker were to stalk Harry, Harry would likely be extremely confused at first. He might try to use magic to fend off the Joker, but the Joker's unpredictability would make it difficult. It could lead to a lot of psychological warfare as Harry tries to figure out the Joker's motives and protect himself and his friends.
In such a fanfiction scenario, Harry Potter's personality would likely be extremely distorted. The Joker's chaotic and immoral nature would probably make Harry grow up with a warped sense of right and wrong. He might be more prone to using cruel and unethical means to achieve his goals, far from the noble and kind - hearted Harry we know from the original series.
In some fanfictions, if Harry Potter becomes a warlock, he might gain even more powerful magic. He could have greater control over spells and be able to perform magic that was previously thought impossible. For example, he might be able to manipulate time in a more complex way or have the power to instantly transport himself anywhere in the wizarding world without the need for a Portkey or Apparition.
If Harry Potter becomes god in fanfiction, he might completely reshape the magical world. He could rewrite the laws of magic, perhaps making it more accessible and less dangerous. He could also bestow extraordinary gifts upon his friends. Hermione might gain the ability to instantly understand and master any spell or bit of knowledge, and Ron could become an unbeatable warrior with superhuman strength in the face of any danger.
In this kind of fanfiction, Harry might teach his children magic in a very different way from how he was taught. He would be extremely protective, having faced so many dangers himself. Maybe he'd pass on the lessons he learned from his own battles, like the importance of friendship and courage.
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.
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.