A Hufflepuff prefect could be the main character. They are responsible for maintaining order within the house and could be caught in the middle of a power struggle between the different houses in this alternate universe. They would have to use their Hufflepuff traits to resolve conflicts and keep the peace in Hogwarts.
One idea could be that in this alternate universe, Hufflepuff is the dominant house in Hogwarts. They have a secret society within the house that holds the key to an ancient magic that can save the wizarding world from a new, unknown threat. And the main character, a Hufflepuff student, discovers this by accident and has to take on the responsibility to gather the members of the society.
Cedric Diggory would be a great main character. He's a well - known Hufflepuff and his values of fairness and hard work would fit well into a story set around a Hufflepuff course.
In an alternate dimension, the main antagonist could be a dark entity that has emerged from the very fabric of the dementions themselves. This entity could be using the Dementors as its minions, controlling them to do its bidding. It would be a powerful and mysterious foe, and Harry would have to discover its true nature and weaknesses in order to defeat it.
It could be an alternate version of Dumbledore. Maybe in this dimension, he was corrupted by power. He uses his vast knowledge and influence to control the wizarding world for his own ends. He manipulates the students at Hogwarts and uses dark magic in secret while maintaining a public facade of being a great wizard.
Severus Snape is a fan favorite in alternate universe stories. People are interested in seeing his character developed further, perhaps in a world where he didn't have such a complex relationship with Lily Potter. Harry Potter himself is, of course, a popular choice. Fanfic writers might create scenarios where Harry discovers his magic much earlier or has different adventures outside of Hogwarts that change his character development.
Perhaps an old Hufflepuff alumni who has turned to the dark side. This person was once a great student but became corrupted over time. They return to Hogwarts with the intention of using the Hufflepuff house as a base for their dark plans. They manipulate the Hufflepuff students, taking advantage of their kindness and hard - working nature, and turn it into a means to an end for their evil deeds. Harry gets caught up in this as he tries to figure out who this mysterious villain is and stop their plans.
It could be a reversed Darth Vader. Instead of being a Sith Lord on the dark side, he could be a powerful Jedi who uses his skills to fight against the new 'evil' which was once good. He might have a red lightsaber still, but it represents the new 'light side' in this reversed universe.
Well, Hermione is often depicted in different ways in these fanfics. In some alternate universes, she could be a powerful dark witch instead of the studious and good - hearted character we know. Ron might be a super - confident and arrogant character rather than the sometimes insecure boy in the original story. Also, Snape could be a completely different person. He could be a jolly and friendly teacher rather than the brooding and complex one we are used to.
It could be alternate versions of the villains from their own universe. For instance, an alternate Voldemort who is even more powerful. Maybe there are also new characters who oppose Harry and Teddy just because they are outsiders in this alternate universe.
Hufflepuff is popular for fanfiction because it represents the underdog. In the books, Gryffindors, Slytherins, and Ravenclaws often get more spotlight. So, writers like to explore the Hufflepuffs' hidden stories. Their traits like loyalty, hard work, and kindness offer a lot of potential for interesting plotlines.