Seamus Finnigan could be a great focus. He's often seen as just a side character, but in fanfiction, he could have his own moment. Maybe he discovers a new spell related to the Triwizard Tournament tasks that no one else knows about. He could use this spell to either help Harry or get himself into trouble while trying to prove himself.
Cho Chang is also underrated. In fanfiction, she could be more involved in the events of the Goblet of Fire. Maybe she has a family secret related to the Triwizard Tournament's origin. This secret could put her in a dangerous position as she tries to balance her loyalty to her friends, her school, and her family's legacy. She could also have her own arc of growth and self - discovery during the events of the tournament.
Cho Chang. In a parallel universe, Cho could be a more central character. Maybe she discovers a new type of magic that is related to her Asian heritage. This could make her a target for both the good and bad sides. She could also have a more complex relationship with Harry or other characters, not just as a love interest, but as a powerful witch with her own motives and goals.
Luna Lovegood is often underrated. In a fanfiction focused on the Deathly Hallows, she could play a crucial role. Her unique perspective and knowledge of the magical world's oddities could help Harry and his friends in unexpected ways. For example, she might have insights into the Elder Wand's true nature that others overlook. Another underrated character is Kreacher. He could be the focus, as his loyalty to the Black family and his knowledge of the old magic could be key in the fight against Voldemort, especially when it comes to the secrets related to the Deathly Hallows.
Luna Lovegood is a great choice. She has such a unique personality and view of the wizarding world. There's so much more that could be explored about her family, her beliefs, and her adventures outside of what we saw in the books. Another is Kreacher. His loyalty to the Black family and his complex relationship with Sirius could be delved into deeper.
Kreacher is another underrated character. He has a complex backstory and a lot of pent - up emotions. In a Deathly Hallows fanfic, he could be the key to unlocking some hidden secrets in the Black family home that are crucial to the fight against Voldemort. Maybe he discovers a long - forgotten spell or artifact that helps the Order of the Phoenix.
Nearly - Headless Nick. He has a lot of knowledge about Hogwarts' history. In a fanfic focused on the Chamber of Secrets, he could be the one to remember an old legend or forgotten fact that helps Harry and his friends. He could also be in danger of being permanently petrified since he's already a ghost, which would add an interesting twist.
Pettigrew. He's often just seen as the traitor, but in fanfiction, you could explore his motives more. Maybe he was forced into his actions by some dark magic or a threat from Voldemort that no one knew about.
Padma Patil. In the original story, she's sort of in the background. But in a fanfic, she could be a really interesting character. Maybe she has some secret knowledge about the Triwizard Tournament or a special connection to one of the visiting schools. She could play a crucial role in helping Harry or another character navigate the challenges of the tournament.
One underrated character is Everett Clopton. You could write about his life before Hogwarts and what led him to be the way he is in the game. Maybe he had a difficult family life that made him so eager to prove himself at school.
Harry Potter himself is of course a very popular character to focus on. Writers can explore his thoughts and feelings during the Triwizard Tournament more deeply. Hermione Granger is also a great choice. Fanfiction might show her researching more about the tournament's history and magic, or her dealing with the jealousy some students feel towards her intelligence. Cedric Diggory is another popular one. You can write about his experiences as a champion, his relationship with his father, and his growing friendship with Harry.
Winky the house - elf. She was briefly in the fourth year story but there's so much more that could be done with her. Maybe she has some secrets about the Dark Arts that she's been hiding because she's afraid of getting in trouble.