He could meet the Joker. The Joker is a well - known character in Arkham Asylum, and his chaotic nature would surely create an interesting interaction with Harry.
Maybe he would encounter Harley Quinn. She has her own brand of craziness and her relationship with the Joker could also play a part in her interaction with Harry. Harry might also run into Scarecrow. With Scarecrow's fear - inducing tactics, Harry would be in for a terrifying encounter.
In this fanfic, Harry could be sent to Arkham Asylum for some mysterious reason. Once there, he might discover secrets about the asylum that the authorities are trying to hide. He could start to question his own sanity as he witnesses the extreme cases of mental illness around him. And perhaps, through his experiences, he might find a way to bring some sort of change or hope to that desolate place.
The Joker could be the main villain. He might see Harry as a new plaything and constantly torment him with his cruel pranks and mind games, making Harry's stay in Arkham a living nightmare.
In such a fanfiction, Harry Potter could bring a new perspective to Arkham Asylum. He has faced dark wizards and dark magic, so he might see similarities in the deranged minds of the asylum's inmates. His Gryffindor courage would lead him to explore the place, perhaps making unexpected alliances with some of the more misunderstood inmates, all while trying to keep his magical identity a secret to avoid chaos.
One possible plot could be Harry exploring the dark secrets of Arkham Asylum. He might encounter various infamous inmates and try to survive their threats while also uncovering a conspiracy within the asylum's walls.
He could meet a young and talented French wizard who becomes his guide in France. This wizard might show Harry all the hidden magical spots in France.
He could meet a Canadian wizard named Oliver who is an expert in ice magic. Oliver could teach Harry how to use magic in the cold Canadian climate.
He could meet the ghosts of his ancestors. These ghosts might offer him some advice or reveal some family secrets that could help him in his future adventures.
He could meet other young wizards and witches who are also on vacation. They might be from different parts of the wizarding world. For instance, a group of wizards from a far - off land with unique magical traditions.
Perhaps he'll meet an old patient who has been in the hospital for a long time. This patient could tell Harry some long - forgotten stories about Hogwarts in the past. There could also be a chance that he meets some other students who are also in the hospital for various reasons, and they might form a new friendship or alliance.
Perhaps he'll meet some interesting magical creatures that are native to the holiday location. It could be a friendly forest sprite or a wise old centaur. He may also cross paths with some international wizards who are also vacationing there and learn about different magical cultures from them.