Well, you could write a fanfiction where the headmaster is secretly training a group of students for a special mission. Maybe there's a prophecy that only the headmaster knows about, and these students are the key to preventing a great disaster. Also, a plot where the headmaster has a past connection to a dark wizard that comes back to haunt Hogwarts would be really engaging. It could involve secrets from the headmaster's youth and how they have to face their own demons to protect the school.
One idea could be a fanfiction where the headmaster has to deal with a new, mysterious dark magic that is affecting the students' dreams. Another could be about a headmaster from a different wizarding school coming to Hogwarts for an exchange program and the chaos that ensues. Or perhaps a story where the headmaster discovers a long - lost spell in the Hogwarts library and has to decide what to do with it.
A great plot could be that Harry decides to open Hogwarts to students from all over the world in a more inclusive way. However, this causes some cultural clashes among the students and staff. He has to find a way to make everyone feel welcome and at the same time maintain the school's traditions. Also, there could be a plot where a mysterious prophecy about Hogwarts is revealed, and Harry as headmaster has to figure out how to prevent its doom.
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.
Perhaps a plot where the headmaster has to deal with a dark threat that is specifically targeting the school in a very different way than Voldemort did. This could involve some ancient magic being misused, and the headmaster has to use his or her wisdom and power to counter it while also protecting the students.
In my opinion, Dumbledore would still be a great headmaster in fanfiction. His wisdom and experience are invaluable. He always had a deep understanding of the students and was able to guide them through difficult times. He also had a great sense of humor and was very approachable despite his great power.
A great plot for a Potter Manor fanfiction could involve a mystery that surrounds the manor. For instance, strange noises at night lead Harry to uncover a long - forgotten chamber. It could also be about Harry restoring the manor to its former glory, perhaps with the help of Hermione's research on ancient magic used in the construction of such manors.
One interesting plot could be Harry dealing with the politics within Hogwarts as headmaster. He might have to face challenges from traditionalist teachers who oppose his more progressive ideas. For example, he could try to introduce new magical subjects that focus on Muggle - wizard relations, which would stir up a lot of debate among the staff and students.
A popular choice could be a character like Sirius Black. If he had survived, he could have become headmaster with his rebellious yet caring nature. He would bring a different kind of leadership to Hogwarts, one that is more in tune with the students' desires for change and adventure.
One interesting plot idea could be exploring what would happen if Harry had a different upbringing, like being raised by the Malfoys. It would completely change his view of the wizarding world and his relationships. Another idea might be a time - travel fanfic where Harry goes back to the Marauders' era and interacts with his parents and their friends. This could lead to him preventing certain tragedies or changing the future in unexpected ways. And then there's the concept of a fanfic where the roles are reversed, and Voldemort is the one trying to protect the wizarding world from a greater evil, with Harry being the main antagonist.
A traveler could be from a different magical realm entirely and come to the Harry Potter universe. They bring with them unique magical abilities that both fascinate and frighten the wizards in the Harry Potter world. This could lead to a series of adventures where they either share their knowledge or are hunted down by those who fear the new magic they possess.
One idea could be that Hecate, being the goddess of magic, takes Harry Potter under her wing secretly to teach him ancient and powerful magic that Dumbledore doesn't know. This could lead to Harry having new abilities to fight Voldemort.