Fanfiction.net is also a great place to search. Just use the search bar and type in 'Joker adopts Harry Potter'. There are likely to be some works there, although the quality may vary. You might have to dig through a bit to find the really good ones.
You can try looking on fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfictions from various fandoms, and there's a good chance you'll find 'Joker adopts Harry Potter' ones there.
One interesting aspect could be how the Joker's chaotic nature would contrast with the magical world of Harry Potter. For example, he might use his wacky sense of humor to disrupt the strict rules at Hogwarts. He could turn Defense Against the Dark Arts classes into his own little shows, much to the dismay of the professors.
Perhaps Bruce Banner adopts Harry Potter after the events at Hogwarts. He takes Harry to the world of science, showing him things like advanced laboratories and scientific research. Harry, in turn, shows Bruce the wonders of magic. Their bond grows as they learn from each other, and together they face new threats that combine elements of both the magical and scientific worlds.
It could be a young wizard who lost their parents during the final battle against Voldemort. Harry feels responsible for all the orphans created by the war and decides to take this particular child under his wing.
The Joker as a professor in Harry Potter fanfiction? That's a wild idea. He could be the professor of Dark Arts in a very different way. Instead of the traditional Dark Arts that the wizarding world knows, he would introduce his own form of 'dark art' which is more about psychological torture and mind games. His classes would be full of his maniacal laughter and unexpected challenges. Students would have to constantly be on their toes, not knowing what he'll do next. He might even use his students in his elaborate schemes against the other professors or the Ministry of Magic.
Well, if we're talking about the Joker from the Batman universe, there's no canonical information indicating he has any knowledge of Harry Potter fanfiction. He's more preoccupied with chaos in Gotham. But in some wild crossovers in fan - created works, he might be depicted as knowing it, just for the fun of mashing up different universes.
I'm not sure if there is a specific instance where a 'Joker' would be directly involved in raising Harry Potter fanfiction. Usually, fanfiction is created and promoted by fans themselves within the fan community, not by a character like the Joker.
Another theme is self - discovery for Harry. With the influence of the MC, Harry gets to explore different aspects of himself. For example, he might discover new magical abilities or gain more confidence in his identity as a wizard. In some fanfictions, the MC helps Harry to look beyond what he thought was possible for himself.
Love and acceptance are huge themes too. Harry has been through so much, and in these fanfictions, the oc shows him unconditional love and acceptance. It's not just about giving him a home, but also about accepting his flaws and his destiny. The oc often has to fight against the prejudices of the wizarding world to keep Harry safe and loved, which adds an interesting layer to the story.
Maybe Aragorn adopts Harry because he sees the potential in him to be a great leader. He takes Harry under his wing to groom him into a leader who can unite different groups, just as Aragorn united the races in Middle - earth. Aragorn would expose Harry to the politics and power dynamics of Middle - earth, and Harry could use his own experiences of dealing with the wizarding world's power struggles to adapt and contribute. Also, Aragorn's strong moral compass would influence Harry to make more ethical decisions in his life.
The kid could be a lost descendant of one of the old wizarding families. Maybe a long - lost relative of the Weasleys or the Blacks. Harry discovers this connection and decides to adopt the kid to preserve the family's legacy.