Harry Potter himself is always popular. In fifth - year fanfiction, readers often focus on his growth and struggles as he becomes more aware of his role in the fight against Voldemort.
One interesting plotline could be Harry secretly learning advanced magic on his own to better prepare for the coming battles. He might discover an old, forgotten spellbook in the Room of Requirement.
One common theme could be criticism of the way Hogwarts was run during Harry's fifth year. Maybe how Umbridge's strict and cruel regime was not effectively opposed by the faculty at first. Another theme might be exploring Harry's isolation more deeply and how his friends could have done better to support him.
Umbridge is very likely to be bashed. She was a horrible teacher and a cruel authority figure in Harry's fifth year. Her actions like using the blood - quill and suppressing the students were despicable.
The headmaster might have a huge impact on Harry's development in his fifth year. He could expose Harry to different magical cultures and theories that are not part of the regular Hogwarts curriculum. This would expand Harry's magical knowledge and way of thinking. Also, the headmaster could help Harry better control his emotions, which are often in turmoil during this year. By teaching him meditation or other mental control techniques, Harry would be better equipped to handle the stress and challenges.
Perhaps it's a former student who was never fully accepted in any of the four houses. They created the fifth house as a place for those who didn't fit in. This person would be very empathetic towards Harry's situation, having been through a similar experience. They would be strict but fair, teaching Harry and his housemates that being different is a strength, not a weakness.
One implication is that it changes the power dynamics in the fanfiction's version of Hogwarts. If Harry is a founder, his ideas and values would have a more direct influence on the school's rules and traditions. For example, there might be more emphasis on the importance of friendship and standing up against evil, which are values Harry strongly represents.
Well, perhaps the headmaster realizes that Harry is being influenced by some dark magic remnants in his fifth year. So, he starts a series of magical purification rituals with Harry. This could involve trips to ancient and secret places within Hogwarts and its grounds. Or, the headmaster could be trying to form an alliance of sorts among the students, led by Harry, to be a united front against the rising dark forces. It might also be that the headmaster is trying to uncover the true motives of some of the new teachers who seem a bit suspicious in Harry's fifth year.
Well, self - discovery is a big theme. Harry is trying to figure out his place in the prophecy and Hermione is constantly learning more about herself and her capabilities. The theme of loyalty also pops up often, as they remain loyal to each other and to their friends in the face of danger. And the theme of facing the unknown, which is very much related to the increasing danger from Voldemort and his followers.
One interesting fifth house Harry Potter fanfiction could be about a group of students who form this new house and they have their own unique set of values and magic. Maybe they are a combination of outcasts from the other houses who come together to create something special.