A great plot could be focusing on the love triangle between Harry, Ginny, and Hermione. Harry might struggle with his feelings for Ginny while also being really close to Hermione, and this could create some interesting drama and tension. Also, it could tie in with the events of the 6th year like the search for Voldemort's Horcruxes.
Another plot idea is to have Harry form an unexpected alliance with Draco Malfoy during their 6th year. Draco is clearly in a difficult situation with his task from Voldemort. Harry could sense his distress and try to help him, which would completely change the dynamic between them and add a new layer to 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 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.
One idea could be Harry and his friends exploring new magical realms that were previously unknown. Maybe they discover a hidden school of magic that has a different approach to teaching spells.
One interesting plot twist could be that a minor character from the previous years turns out to be a key figure in the fight against Voldemort. For example, a Hufflepuff student who was always in the background might have some secret knowledge or power that helps Harry in his final battle.
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.
One idea could be to focus on Harry's relationship with Sirius Black in a more in - depth way. Maybe Harry discovers some hidden secrets about Sirius that even Sirius himself doesn't know yet. Another could be to create a new magical creature that is somehow related to the events of the third year, like a creature that is connected to the time - turner.
Well, a great plot for a Harry Potter third year fanfic could be centered around time - travel. Since Hermione has the Time - Turner in the third year, what if there was a glitch in it that sent some students back in time to the Marauders' era? They could interact with James, Sirius, Remus, and Peter, and this could change the course of events in the present. Also, a story where Harry starts to suspect that there is more to the Dementors than just being guards of Azkaban. He could start his own investigation, which would lead him to some dark secrets about the wizarding world's prison system.
One idea could be exploring more of the backstories of the other schools' students who came for the Triwizard Tournament. For example, delving deeper into the life of a Durmstrang student and how their magic education differs from Hogwarts.
One interesting plot could be about Harry forming an unexpected alliance with some Slytherin students during his 5th year. They might team up to uncover a mystery within Hogwarts that involves a dark magic item that was planted there by Death Eaters. This could lead to a lot of character development as they learn to trust each other despite their house differences.