Well, perhaps Harry is tired of all the chaos in the wizarding world. Maybe he decides to go on strike against the Ministry's unfair rules. He could be striking for better treatment of house - elves or more equality among wizards of different blood status.
Obviously, Harry himself is the central character. Hermione would probably be involved too. She might try to reason with Harry at first but then end up supporting him. Ron would also be there, likely being a bit more hesitant but still standing by his friends.
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.
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.
He might meet the house - elves that have been taking care of Potter Manor for generations. They could show him around the manor and tell him stories about his family.
There could be other Hogwarts students. For example, Neville could be there, exploring the different plants and herbs that are associated with Beltane. Luna Lovegood might also make an appearance, with her unique perspective on the magical aspects of the festival. And perhaps some local wizards and witches from the area where the Beltane celebration is taking place.
The main character is Harry. But in Pandora, he would meet a variety of characters. There could be a young Na'vi who becomes his guide, showing him around Pandora. Also, there might be a human scientist on Pandora who teams up with Harry. This scientist could be interested in the magical aspects Harry brings and together they could face the challenges in Pandora, both from the natural elements and the power - hungry humans.
Well, perhaps Harry could discover the unique flora and fauna on Pandora. He might meet the Na'vi and learn their ways, like how to ride a banshee. And he could use his magic in unexpected ways in Pandora's environment.
In the 'Harry goes to the hospital fanfic', it's possible that Harry is injured during a Quidditch match. He is then rushed to the hospital. There, he could discover a hidden part of the hospital that has some ancient magic stored. He might also have some deep conversations with his friends who come to visit him about their future plans and the current state of the wizarding world.