The Weasley family could also be part of it. Ginny, for example, could be involved in some way, either as a friend or a more central figure if the plot involves some kind of threat to Hogwarts. And then there are the Death Eaters or some other dark wizards who might be the antagonists if the story is about the champions coming back to fight against a new dark force. There could be some old, forgotten characters from the wizarding world's history that are somehow connected to the champions' return as well.
Obviously, the previous Triwizard champions like Cedric Diggory (even though he passed away, there could be some magical way he's brought back), Fleur Delacour, and Viktor Krum would be the main characters. Harry Potter himself would also be involved as he was an unexpected champion in his time.
Harry Potter himself would most likely be one of the main characters. Since it's related to the Harry Potter universe, he would be at the center of things. Hermione and Ron would also probably be involved, as they are his closest friends. Additionally, characters like Dumbledore might make an appearance, even if he was dead in the original series. His wisdom could be crucial in dealing with the situation of the dead returning.
It's hard to say without reading the fanfiction. They could be the Triwizard champions like Cedric Diggory, Fleur Delacour, and Viktor Krum. Or it could be some new champions created specifically for this fan - made story.
It could also center around a mystery related to their previous participation in the Triwizard Tournament. Something that was left unfinished or a secret that's now starting to surface. Their return might be triggered by signs or prophecies related to this mystery. They would then have to work together, along with Harry Potter and his friends, to solve it and prevent any potential disasters in the wizarding world.
Cedric Diggory is often quite popular. Since he was a champion in the original series, fanfiction writers like to explore more of his story, his thoughts during the tournament, and even what could have happened if he had survived. He has that heroic and noble quality that makes him a great focus.
The younger students at Hogwarts are quite likely to read Harry Potter fanfiction. They are at an age where they are really into the wizarding world and would be curious about different interpretations. Students like Colin Creevey, who is a big fan of Harry, might be eager to read fan - created stories about his hero. Also, the members of Dumbledore's Army, such as Luna Lovegood and Neville Longbottom, seem like the types who would be open to exploring fanfiction.
Fleur Delacour is quite popular too. Her beauty and her status as a Beauxbatons champion make her a fascinating character to write about. Many fanfictions explore her relationship with the other champions, especially Harry. Additionally, Hermione Granger often plays an important role in these fanfictions. Her intelligence and her friendship with Harry mean she can be involved in many different storylines related to the Triwizard Tournament, like helping the champions prepare for their tasks.
Cedric Diggory would be a likely champion to defend Harry. He is from Hogwarts too and is known for his fairness and good nature. He might defend Harry because he realizes that Harry is in a difficult situation and doesn't deserve the extra hardships that are being piled on him.
The returning champion is obviously a main character. They are the center of the story's action, driving the plot forward with their comeback.
Hermione is a very likely candidate. With her intelligence, she could easily start a business. For example, she might open a library - like establishment but with rare and restricted magical books that she has managed to collect through her research. She could charge a membership fee for witches and wizards to access these valuable resources.
In addition to the main ones, there could be some original characters created for this fanfiction. For example, a wizard who is secretly working for the Queen's enemies. There might also be some magical animals that are either helping Harry or causing trouble. And of course, the Queen's advisers and guards would be part of the story as they interact with Harry and the magical world.