Perhaps Ginny becomes friends with one of the foreign students who come for the Triwizard Tournament. Through this friendship, she and Harry learn about different magical cultures. This new knowledge then helps them in some unexpected way during the tournament. At the same time, their relationship evolves as they share these new experiences.
One idea could be that Ginny starts to show her true magical potential during their fourth year. Maybe she discovers a new spell that helps Harry in a Triwizard Tournament task. Another plot could be that Harry and Ginny have to work together secretly on a project related to the tournament, and during this time, their feelings for each other start to grow more deeply.
One idea could be Ginny trying to find the courage to talk to Harry for the first time. Maybe she writes him a secret note but is too shy to give it to him directly. Another plot could be them accidentally getting paired up for a Hogwarts task and having to work together, which helps break the ice between them.
A plot could revolve around Ginny getting involved in a secret club or group that Harry is also a part of in their fifth year. This could lead to them having adventures together, exploring hidden parts of Hogwarts. And perhaps they find a mysterious artifact that has some connection to the greater battle against Voldemort. Their relationship could be tested and strengthened through these events.
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 possible plot is that there's a mysterious illness spreading among the Quidditch players, and Harry and Ginny have to figure out what's causing it and how to stop it before the big Quidditch tournament. Or perhaps a new and dangerous Quidditch move is introduced by a visiting team, and Harry and Ginny must learn it quickly to stay competitive. They could also have a friendly rivalry with another couple who are also big Quidditch fans.
Perhaps there's a magical guitar that Harry and Ginny find. It has special powers, like when Ginny plays it, it can calm the most ferocious magical creatures. They go on a journey to figure out the origin of the guitar and along the way, their relationship deepens as they face various challenges together.
One idea could be that Harry and Ginny are swimming in a secret pool at Hogwarts that has some magical properties. Maybe the pool can change its shape or temperature based on their emotions. Another could be that they are having a swimming race to train for a Triwizard Tournament - like event for adults. And they use magic to enhance their swimming abilities in creative ways.
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.
One plot idea could be Harry struggling with his newfound alcohol problem while also dealing with the Triwizard Tournament. Maybe his drinking starts as a way to cope with the stress and danger, but it quickly gets out of control and affects his performance in the tasks. Another could be his friends trying to stage an intervention, which causes a rift between them as Harry doesn't want to admit he has a problem.
One plot idea could be that Hermione and Ginny team up to play a prank on Harry, but it backfires in a hilarious way. For example, they try to make his hair turn purple but instead it starts changing colors every few seconds. Another idea is that the three of them go on a secret adventure outside of Hogwarts to find a lost magical artifact. They face various challenges and their friendship is tested. And perhaps a plot where Hermione discovers a new spell that accidentally causes some chaos between Ginny and Harry's relationship, and she has to find a way to fix it.