The main plotlines could include their joint quest to protect the Goblet of Fire from evil forces. Maybe it shows how their relationship evolves as they deal with the pressure and expectations of the tournament. Or, it could feature unexpected allies and foes emerging because of their newfound sibling connection.
Perhaps a fanfiction where Harry is angry due to the danger the Goblet of Fire has put his friends in. He realizes that just by being a part of the Triwizard Tournament, his enemies might target his friends to get to him. His anger could drive him to take extra precautions to protect them, and also make him more determined to succeed in the tournament so that he can prove that he's not a coward and that he can handle whatever comes his way.
It could be a story exploring their sibling relationship and adventures in the magical world. Maybe it focuses on Harry protecting and guiding his sister through various challenges.
One interesting plotline could be Draco secretly helping Harry during the Triwizard Tournament. Maybe he has a change of heart and realizes that the dark side isn't what he really wants. Another could be a friendship forming between them in the face of a common enemy within the tournament. For example, a dark wizard who tries to manipulate the tasks for their own evil purposes.
One possible plotline could be a friend betraying Harry to gain favor with the Death Eaters. Maybe they leak information about Harry's tasks in the Triwizard Tournament. This would add a whole new layer of danger and drama to the story.
Well, some fanfictions might focus on alternate timelines where Harry makes different choices. Others could involve him facing new magical threats or forming unexpected alliances.
One great fanfiction could be where Harry feels alone during the Triwizard Tournament. Maybe he's isolated because of the jealousy and suspicion from other students. He might find solace in the Forbidden Forest, encountering some magical creatures that help him see his situation differently.
Well, you might find fanfics where they team up to solve mysteries within the wizarding world or face off against powerful dark wizards. Another common theme is their relationship evolving and growing stronger over time.
In some fanfictions, Snape could be the key to unlocking the mystery behind the Goblet of Fire's malfunction. Perhaps he discovers a dark magic spell that was cast on the Goblet long ago and has to race against time to reverse it. Along the way, he has to interact with Harry and the other students in ways that he normally wouldn't, which could lead to some very interesting character development.
One interesting fanfiction idea could be Harry Potter as an adult going back in time to the Goblet of Fire events. He could use his more advanced magical skills and knowledge to perhaps prevent some of the more dangerous situations that occurred. For example, he might be able to better protect Cedric Diggory from the start.
The story could focus on how the brother copes with the expectations and comparisons that come with being related to the 'Girl Who Lived'. It might also feature their joint efforts to face off against dark forces or discover long-lost magical secrets.