Perhaps in the fanfiction, we could see a different side of the foreign students. Like, the Beauxbatons and Durmstrang students could have their own hidden agendas. One of them might be in love with Harry and tries to protect him during the tournament tasks. This would create a whole new dynamic of relationships and loyalties within the story.
Another interesting plot could be that Harry's connection to Voldemort causes some strange magical disruptions during the Triwizard Tournament. These disruptions seem like accidents at first, but Harry gradually realizes they are being caused by his own dark - side related powers that are being awakened in his fourth year.
A great plot for a second - year fanfiction could involve the basilisk in a different way. Maybe it wasn't just Ginny who was being influenced by Tom Riddle's diary. What if there were other students who were also subtly affected, and Harry and his friends had to figure out who was really in danger and who was a potential threat? It would make for an exciting adventure.
One idea could be exploring a different version of the Triwizard Tournament. For example, what if there were more magical creatures involved in the tasks? Another could be focusing on Harry's relationship with Sirius during that year in a more in - depth way, like Sirius secretly guiding Harry through some of the tournament challenges. Maybe also a plot where a new student arrives who has a connection to Voldemort's past that no one is aware of and they try to disrupt the tournament.
A fanfiction could also be about Harry's relationship with his parents during his fourth year. Through some magical event, he gets to communicate with their spirits or memories more vividly. This could help him understand his parents better and also give him strength and guidance for the challenges he faces in that year.
A really cool plot idea would be if there was a new magical creature introduced in Harry's third year that was related to the Dementors in some way. Harry could have to learn about this creature to better understand how to combat the Dementors and it could add a whole new layer to the story.
Another plot could revolve around Harry's Occlumency lessons. In fanfiction, it could be that instead of just trying to block his mind from Voldemort, he actually manages to use Occlumency to his advantage. He could start to plant false thoughts in Voldemort's mind through their connection, leading Voldemort astray in his plans.
One idea could be that Harry discovers a new, hidden magical ability within himself that helps him in the final battle against Voldemort. For example, the ability to communicate with the spirits of Hogwarts founders for guidance. Another could be that the relationships between the characters take a different turn. Maybe Hermione and Ron have a huge fight early on and Hermione teams up with Draco for a while out of necessity, which changes the dynamics of the whole group. Also, Neville could have an even more prominent role, leading a secret group of students who are training in a new form of magic that they hope will give them an edge in the fight.
A plot could be that Harry has premonitions about the Triwizard Tournament tasks before they happen. These premonitions are not clear at first, but as he gets closer to each task, he starts to understand them better. It adds an element of mystery and foreshadowing to the story.
A plot idea could be that Neville is forced by some dark magic to betray Harry. Neville has always been a bit of an outcast and maybe some dark wizard finds a way to use his insecurities against him. He could leak some of Harry's plans regarding the Triwizard Tournament tasks, which would put Harry in great danger.
Maybe in this fanfiction AU, the relationship between Harry and Hermione develops romantically during the fourth year. This would change the dynamics of their friendship and how they interact with Ron. They could face challenges together in the tournament, relying on their newfound feelings to get through difficult situations.