One unexpected character could be Firenze. Since he is a centaur, his different perspective on magic and his knowledge of the stars could bring a unique element to the tournament. He might be a competitor, or perhaps he is a special advisor to the organizers. And then there's the portrait of the Fat Lady. What if she could come to life for the tournament? She could be a wildcard character, using her knowledge of the students and Hogwarts secrets to either help or hinder the participants.
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.
Well, perhaps a character from a very low - key anime that no one expects. Like a character from a slice - of - life anime who has some hidden power. Maybe a schoolgirl who can control time in short bursts, but has been keeping it a secret in her normal life.
How about a group of aliens who have heard about the One Punch Man tournament and are curious to see the power levels on Earth? They come with their own set of powers, like the ability to transform their bodies into different elements. One of them might be able to turn into water, making it difficult for the heroes to land a solid punch. These aliens add a whole new level of chaos and excitement to the tournament as the heroes have to figure out how to fight against such strange and powerful opponents.
Hermione could also be a main character. Maybe she uses her intelligence to come up with new strategies for archery or to solve problems during the tournament. Ron could be there too, providing comic relief and also being a loyal friend to Harry during the tournament.
A wizard from a far - off land, like a wizard from a hidden tribe in the Amazon rainforest. He or she could bring a completely different style of magic that the Hogwarts students and other champions have never seen. Their magic could be more nature - based, using the power of the rainforest plants and animals. This would make their approach to the Triwizard tasks very different and exciting.
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.
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.
A young wizard from a nomadic wizarding tribe. This character could have skills in magic related to the elements in a very unique way. For instance, they can control the wind to fly without a broomstick, which would be amazing in the Triwizard Tournament tasks. They would also bring in a different cultural aspect to the story with their tribal traditions and beliefs.
A dragon - tamer from a nomadic magic tribe. This character could have grown up around dragons and have a deep understanding of their behavior. They could be a wild card in the tournament, not really following the traditional Hogwarts ways of dealing with dragons and bringing a fresh perspective.
Some unexpected characters for the crossover could be the Powerpuff Girls. They could use their superpowers to help protect Hogwarts from dark forces. Then there's the Mad Hatter from Alice in Wonderland. His insanity and unique view of the world could bring a new perspective to the characters in Harry Potter. Also, Godzilla. Although it seems very out of place, his massive size and destructive power could be a threat that the wizards have to deal with in a very different way than they're used to.
Genos could be an unexpected opponent. He might be forced to fight Saitama due to some mind - control or a misunderstanding in the rules of the tournament. It would be really interesting to see how Saitama would handle fighting his own disciple, and how Genos would fare against his master's incredible strength.