For a more complex plot, a crossover with 'Lord of the Rings' would be fascinating. Harry could find himself in Middle - earth and have to use his skills from the wizarding world to help Frodo on his quest. The Goblet of Fire could be related to some dark magic in Middle - earth, and Harry has to figure out how to deal with it.
A cool plot could involve a crossover with 'Star Wars'. Picture Harry using the Force in addition to his magic while competing in the Triwizard Tournament. Or perhaps a crossover with 'Alice in Wonderland' where the Goblet of Fire sends Harry down the rabbit hole and he has to navigate Wonderland's strange rules during the tournament.
One idea could be introducing a new character from another universe who accidentally gets transported into the Goblet of Fire timeline. This character could have unique magical abilities that both fascinate and worry the Hogwarts students. For example, they might be able to control time in a different way than Hermione's time - turner. Another plot idea could be that Voldemort discovers a new dark magic spell from this other universe through a mysterious object. This could make him even more powerful and the trio has to find a way to stop him using knowledge from both universes. Also, what if the Goblet of Fire chooses a student from a completely different magical school in this crossover? It would create a whole new dynamic of competition and friendship.
One idea could be that Harry has a group of friends from different houses who are all secretly in love with him. They start to form a sort of harem around him during the Triwizard Tournament. For example, Hermione could be using her intelligence to help him in the tournament in a more personal way, and Cho Chang might be more openly affectionate, and then there could be a girl from Beauxbatons who is also drawn to Harry's bravery.
One idea could be that the OC is a foreign exchange student who has unique magical abilities unknown to Hogwarts. They could help Harry in the Triwizard Tournament tasks in unexpected ways, like using ancient spells from their home country.
One idea could be exploring what would happen if a different student was chosen by the Goblet of Fire. Maybe a Muggle - born student who has to face extra prejudice during the Triwizard Tournament. It would add a new layer of conflict and show how the wizarding world reacts to a 'less likely' champion.
One interesting plot idea could be exploring what would happen if a different student was chosen for the Triwizard Tournament. Maybe a shy and overlooked student gets picked, and they have to overcome not only the tournament tasks but also their own insecurities. Another idea could be a love triangle involving characters that were not part of one in the original story. For example, a friendship between a Hufflepuff and a Slytherin turns into a complex relationship when feelings for Harry Potter get involved.
One idea could be creating an OC (original character) who is a foreign exchange student at Hogwarts during the Triwizard Tournament. This character could bring unique magical traditions from their own school or country, and get involved in the events of the Goblet of Fire in unexpected ways.
One idea could be exploring an alternative version of the Triwizard Tournament. For example, what if the tasks were designed by a dark wizard who had infiltrated the Ministry of Magic? This would add a whole new level of danger and mystery to the story. Another idea could be focusing on a character other than Harry in the tournament. Maybe Neville Longbottom gets chosen by the Goblet of Fire and has to face his own fears and insecurities throughout the tasks. It could show his growth and transformation into a more confident wizard.
One interesting plot twist could be a character who was thought to be a friend actually being in league with the Death Eaters all along. For example, a minor character like Seamus Finnigan could be secretly working for Voldemort, passing information about Harry's movements. This would add a whole new layer of danger and suspicion to the story.
A great plot twist could be that the Goblet of Fire was actually tampered with by a group of former Death Eaters who wanted to bring back Voldemort in a different way. They planned to use the tournament as a cover to perform a dark ritual, and Harry uncovers this plot halfway through the story.
Well, a plot could involve Harry and Hermione being forced to enter the tournament together as a pair, against the rules. They then have to face challenges that are not only dangerous but also designed to test their relationship. For example, in one task, they might have to choose between saving each other or winning the task. This would create a lot of drama and tension between them. Also, there could be a sub - plot where they start to realize their feelings for each other during the tournament.