Hermione Granger could also be a great main character for this type of fanfiction. Her intelligence and determination would make her attempts to understand and befriend an enemy very engaging. She could use her knowledge of magic and her deductive skills to figure out what the enemy is up to while getting closer to them.
Harry Potter himself could be the main character. Given his history with Voldemort and his Death Eaters, it would be interesting to see him try to get closer to an enemy to gain an upper hand in the fight against the Dark Arts.
Perhaps the fanfiction could explore how keeping an enemy closer affects the character's relationships with their friends. They might have to keep secrets from their friends, which could lead to some tense and interesting situations. For example, if Harry Potter were to befriend Draco Malfoy secretly, Ron and Hermione would be very suspicious and it could create a rift in their trio.
Voldemort and his Death Eaters would still be the main enemies. But now, Harry's guns would pose a new threat to them. They might not be used to dealing with such Muggle - like weapons, even if they are magically enhanced.
There could be a new dark wizard or witch who is jealous of the Potter family's legacy. They might target Harry Potter's daughter to try and bring down the family name and prove their own power.
The Death Eaters could still be a major threat. Even with Voldemort gone, some of them might be trying to revive the dark ways and see the wizard king as a big obstacle in their path.
A lesser - known character like Ernie Macmillan from Hufflepuff could be the main character. As a prefect, he could bring out the unique qualities of Hufflepuff house. He might have to deal with issues like other houses underestimating Hufflepuff students. His story could focus on how he promotes the values of hard work, loyalty, and fairness within the house and the school as a whole.
Hermione could be a great main character. She's curious and intelligent, so she would be really interested in exploring the secrets of the diary. She could use her vast knowledge of magic to try and figure out how the diary works and what secrets it holds.
Harry Potter himself could be the main character as a revenant. Maybe he has some unresolved issues from his past, like his relationship with his parents. His revenant form could be on a journey to fully understand their sacrifice and his own place in the wizarding world's history.
It could be a younger sibling of one of the main characters in the original series. For example, Ginny's little sister who has big shoes to fill but also a great passion for Quidditch.
Hermione could be a great main character in an elementalist fanfiction. Her intelligence and thirst for knowledge would make her eager to learn about and master elemental magic. She could be the one to discover a long - lost elemental spell book and start exploring the powers.
It could be a new student at Hogwarts who has a natural talent for dealing with beasts. This student could be from a family that has always had an affinity for magical creatures but has kept it a secret until now.