Another possibility is that it's a descendant of an ancient wizarding family. This character has been sheltered from the wizarding world until they discover their second magical core. Their family's secrets and history could be intertwined with the nature of this second core, and they have to navigate the wizarding world while also uncovering their family's past.
Neville would also make an excellent main character. He has a natural affinity for plants, and magical creatures are not that far off in the wizarding world. He could be the one to discover a new way to care for ailing magical creatures, perhaps using his knowledge of herbology. For example, he might find a plant that can cure a disease that's been affecting the unicorns in the forest.
One interesting idea could be the exploration of a new magical power source. It might introduce unique spells or abilities related to this second magical core. For example, the character could have a different type of magic that is not found in the original series, like the ability to communicate with magical creatures in a more profound way.
It could be a young witch or wizard who is a transfer student to Hogwarts. They would be the main character as they try to adapt to the new environment and in the process, discover a magical mystery that sets them on a journey.
Harry Potter himself is often a popular character in such fanfiction. His already - powerful magical core being altered can lead to all sorts of interesting storylines. Hermione Granger is also a favorite. Fans like to explore how her brilliant mind would deal with an altered core, whether it enhances her intelligence - related magic or gives her new challenges. Draco Malfoy is another one. Altering his core can create a whole new dynamic for his character, from his relationship with his family to his standing in the wizarding world.
In Harry Potter magical core fanfiction, there are often ideas about rare magical cores. For instance, a dragon heartstring core from a very ancient and powerful dragon could be a central concept. This might give the wand wielder an edge in combat magic. There could also be fanfictions that deal with the malfunction or corruption of a magical core, leading to unforeseen and dangerous magic. Some might even create new types of magical cores based on mythical beasts not mentioned in the original series, like a basilisk fang core which could have venom - related magic abilities.
It could be a corrupt wizard within the Ministry of Magic's financial department. He might be embezzling funds and using dark magic to cover his tracks. When Harry starts to dig into the strange financial situations in Magical Britain, this wizard tries to stop him by all means, which eventually leads to the bankruptcy as the situation gets out of control.
It could be a new student at Hogwarts. This student comes from a family that has a long - hidden knowledge about the special magical forces. They enter Hogwarts and start to share this knowledge with their new friends, causing a stir in the school. Neville could also be a main character. He has grown so much throughout the series, and he could use his newfound confidence to explore these special magical forces and prove himself even more.
There could also be a group of pure - blood wizards who look down on Harry's connection with magical creatures. They see it as beneath them and try to undermine his position as the king of magical creatures. They might spread false rumors about him and try to turn other wizards against him.
A time - traveling wizard from a far - off future in the magical empire. This character could bring knowledge of advanced magic that has not been discovered yet in the current time of the Harry Potter universe. They might be on a mission to prevent a future disaster from occurring in the past, but their actions could have unforeseen consequences on the present - day magical empire.
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.