Voldemort could still be the main antagonist. Even though Harry is alone, Voldemort would see him as a threat and continue to send his Death Eaters after him. Harry would have to face these attacks all by himself, which would make the story very intense.
One possible storyline could be Harry feeling isolated and betrayed by his friends at Hogwarts. He might decide to go off on his own adventures, exploring parts of the wizarding world that are less known, like some hidden magical forests or ancient ruins.
One possible plot could be that after the events of the fourth year, Harry discovers that some of his friends and allies are starting to doubt him because of the return of Voldemort. This makes him feel isolated. He might then start to focus more on self - study of magic, exploring hidden parts of Hogwarts on his own.
In some fanfictions, Harry might become more withdrawn and self - reliant. He could start to question his place in the wizarding world even more deeply, perhaps feeling that he has to face the upcoming threats on his own. He might also begin to explore his magical abilities in isolation, away from the watchful eyes of Hogwarts professors.
One possible storyline could be Harry feeling isolated as he prepares for the Triwizard Tournament without much support from his friends. Maybe Ron is jealous and Hermione is too busy with her own studies. So Harry has to rely on his own skills and instincts more, like when he's figuring out the clues for the tasks.
Hermione could be a main character. She might be one of the few who still believes in Harry and tries to reach out to him despite his isolation. Another could be Dobby. Dobby, being the kind-hearted house - elf, could bring Harry food and supplies while he's in isolation.
There might be a fanfiction where in their fourth year, Harry and Hermione go on a secret adventure related to the Dark Arts that they suspect is connected to Voldemort's return. Along the way, they face many challenges and have to rely on each other's strengths. Hermione's knowledge of spells and Harry's courage are put to the test, and this experience brings them closer together as friends and perhaps something more.
A really engaging one is 'Year Four Revelations'. In this fanfic, the author explores the relationships between the characters during Harry's fourth year. It shows a different side of how the students interact and deal with the events of that year. There are some great character - building moments and it's very immersive.
Well, since Harry is alone, he might view Cedric as more of a rival than he would if he had his friends around. He could be more suspicious of Cedric's motives in the Triwizard Tournament. Maybe he doesn't trust Cedric as much and is constantly on guard around him.
Harry's character may develop a sense of mystery. Since he's allone, others don't know what he's up to. He might be exploring parts of the wizarding world that are not well - known. He could also become more brooding, constantly thinking about the challenges ahead and how to face them alone. He may develop new skills in secrecy, and his confidence could grow in a different way compared to when he was at Hogwarts surrounded by friends and mentors.