It could be Luna Lovegood. She has a unique perspective on magic. In the final battle fanfiction, she might use her strange but effective magical items and her unorthodox thinking to outwit Voldemort. For example, she could use some of her Lovegood family's special magical artifacts to create a shield or a trap for the Dark Lord.
Happy. He's usually seen as a sidekick, but in a fanfiction, he could be the most unexpected hero. Maybe he discovers a new form of his flying magic that can disrupt the enemy's defenses on a large scale.
The house - elves could be unsung heroes. In fanfiction, they might use their magic in ways that the wizards and witches don't expect. Since they are so good at household magic, they could turn that to their advantage in the battle. For instance, they could make the Hogwarts castle even more of a maze for the Death Eaters to navigate, or they could use their skills to quickly repair any damage done to the castle's defenses.
In a Harry Potter final battle fanfiction, Harry might use some new and unexpected spells. For example, he could combine the Patronus Charm with a powerful offensive spell to repel the Death Eaters in a more spectacular way. Maybe he discovers a hidden power within himself that gives him an edge in the fight.
In many fanfictions, after the final battle, Harry might focus on rebuilding Hogwarts. He could use his influence to make it a more inclusive and modern place of magic learning, with new courses perhaps on Muggle technology and its integration with magic.
Maybe Dobby could be an unexpected ally. He could use his magic to create distractions for the other contestants or help Harry find a hidden passage in the maze. Another possibility is that one of the Hogwarts ghosts, like Nearly Headless Nick, decides to assist Harry. He could float through walls and spy on the other champions' movements and relay the information to Harry.
Filch could be an unexpected character. He might enter the tournament in a comical way, like he accidentally stumbles into the selection process and ends up being a participant. Another could be Peeves. He could disrupt the tournament in his usual mischievous way, or perhaps he is made a part of a task where the competitors have to outwit him.
Obviously, Harry Potter is one of the most important. His role as the Chosen One is central to the final battle. Hermione Granger also plays a crucial part with her intelligence and knowledge of magic. Ron Weasley too, as he provides support to Harry.
Well, one possibility is that Harry, Ron and Hermione would focus on rebuilding Hogwarts. They might use their magic to restore all the damaged parts of the castle, making it even more magnificent than before. And they could start new magical education programs to ensure that future wizards and witches are well - trained in both magic and ethics.
Ron Weasley could also be the partially kissed hero. In some fanfictions, Ron might have a moment with a girl where they start to kiss but are interrupted. He's a character who has his own share of romantic interests in the series, and this could be an interesting take on his character in fanfiction.
Obviously, Harry Potter himself would still be a central character. He would be dealing with the aftermath of the battle, like post - traumatic stress or trying to bring about positive changes in the wizarding world.