One way for Harry Potter to save the Joker in fanfiction is through time - travel. Harry could go back in time to prevent the events that made the Joker who he is. Or he could take the Joker to a different time period in the wizarding world where the Joker could see a society that functions without the same level of crime and corruption. This new perspective might inspire him to change his ways.
In a fanfiction, Harry Potter might save the Joker by transporting him to Hogwarts. The magical environment and the strict rules of the school could be a shock to the Joker. Harry could introduce him to the concept of friendship and loyalty as shown by the Hogwarts houses. The Joker might start to change as he experiences a different kind of life away from the crime - filled streets of Gotham.
Well, it could happen in a world where magic and the chaos of Gotham collide. Maybe Harry uses his magic to get into the Joker's mind and find the good that's left in him. He could use a spell like Legilimency to understand the Joker's past traumas and then use healing magic to start the process of redemption.
Well, in a fanfiction world, anything can be possible. Maybe Harry Potter uses his magic skills like the Killing Curse. But it's really a strange crossover. The Joker is from a completely different universe with no magic, and Harry Potter is from a world full of magic. It would be a very unique and perhaps disturbing story to see how Harry Potter overcomes the Joker's chaos and madness with his magic and finally kills him.
Well, in this fanfiction scenario, Harry Potter might use his magic skills. Maybe he travels through time and space using a spell like the Time - Turner or some other powerful magic. He could then arrive in the Star Wars universe where Shmi is in danger. He'd use his spells to fend off the attackers, like using a Stunning Spell on the Tusken Raiders who might be threatening Shmi.
Well, perhaps there is a dark force that is after Maximoff and it's affecting different universes. Harry Potter, being the hero he is, senses this disturbance in the magical fabric. He travels to Maximoff's realm using his Patronus as a guide. Once there, he uses his knowledge of defensive magic, like the Shield Charm and Stunning Spells, to protect Maximoff and save him from the impending doom.
One way this could happen is if Harry Potter discovers a new form of magic that can reverse the dark magic influencing the Death Eaters. Maybe he finds a powerful spell during his exploration of ancient magic that can purify their souls and free them from Voldemort's control.
In a fanfiction scenario, it could be that the Time Stone has some sort of connection to the magic in the Harry Potter world. Doctor Strange is in trouble while dealing with an enemy who wants to misuse the Time Stone. Harry Potter, with his knowledge of time - turning devices like the Time - Turner, manages to outwit the enemy and save Doctor Strange.
In a fanfiction scenario where Harry Potter knows the Joker, it would be a very interesting crossover. Harry, being used to a certain code of ethics in the wizarding world, might be shocked by the Joker's complete lack of moral boundaries. He could attempt to reform the Joker using magic, perhaps a spell to make him see the error of his ways. On the other hand, the Joker might see Harry as just another plaything in his game of chaos and try to break Harry's spirit.
Well, it could be a total chaos. Harry Potter's magical world is all about order in a way, with Hogwarts rules and the fight between good and evil in a more traditional sense. The Joker, on the other hand, is the agent of anarchy. Maybe he'd start causing mayhem in Hogwarts, like turning the moving staircases into death traps or making the portraits go berserk. And Harry would have to use his magic in really unorthodox ways to stop him.
The plot could start with Harry somehow ending up in Gotham. He notices the Joker's chaos and decides to intervene. Harry might use his invisibility cloak to get close to the Joker without being detected at first. Then he could start using his magic to slowly disrupt the Joker's evil plans.
It could be part of a plot where Draco is given an assignment by someone in the wizarding world to save Harry from the Dursleys. He initially does it for self - serving reasons like getting in good with certain powerful wizards who are concerned about Harry's well - being. But during the process, he actually starts to care about Harry.
In some fanfictions, Hermione might use her intelligence and magic skills. For example, she could use a powerful spell to create a distraction at the Dursleys' place. Maybe a spell that makes all the dishes in the kitchen start flying around, causing chaos. Then, while the Dursleys are distracted, she could quickly grab Harry and apparate away to a safe place, like the Burrow or a hidden wizarding location she has prepared in advance.