Well, one possibility could be that the story explores the conflict of her being related to Makorov while having different values. Maybe she tries to redeem her family's name. She could use her magical abilities (assuming Harry still has magic in this AU) to fight against the dark deeds associated with Makorov's past.
Her relationships with her friends would likely be strained at first. They might be suspicious of her due to her relation to Makorov. But if she shows her true self, they could come around. For example, Hermione might be initially hesitant but then realize Fem Harry is still the same person deep down.
One interesting fanfiction could be 'The Caius Connection'. In it, Fem Harry's journey as Caius' granddaughter unfolds with her learning about the vampire world in a unique way. It has great character development and an engaging plot.
One possible plot twist could be that Fem Harry's identity as Makarov's granddaughter is initially kept a secret from the main characters. This could lead to a situation where she is unknowingly working against her own family's interests at first. For example, she might be involved in a mission that is actually aimed at undermining Makarov's plans, but when her true identity is revealed, it throws everyone into chaos.
The novelty of it. Combining the world of 'Harry Potter' with the 'Twilight' universe through this unique relationship is really exciting. It allows for a fresh take on both universes.
His relationship with the Dumbledore family could be more complex. Since he's reborn as a granddaughter, Dumbledore's legacy might be a heavier burden or a guiding light in a different way. Also, relationships with teachers at Hogwarts would change. They might treat her more gently at first, but as she shows her true abilities, they'd have to reevaluate how they interact with her. And family relationships would be completely new. She might have a different family dynamic that she has to navigate, which could be both challenging and rewarding.
Her character could develop through self - discovery. As she learns more about her relation to Makarov, she starts to question who she really is. This could lead her to explore different aspects of her personality that she had previously ignored. For instance, she might find that she has a more ruthless side like Makarov, but she tries to temper it with her own sense of justice.
It could be a story where Harry's granddaughter's rebirth is a result of a powerful dark magic spell gone wrong. When she is reborn, she has a unique connection to the dark side of magic. She has to fight against the influence of this dark connection while also trying to find her place in the wizarding community again. Along the way, she meets new friends and allies who help her in her journey.
Maybe the fanfiction could have a plot where the wizarding world is in danger again, and the granddaughter, with the help of Harry's memories or guidance (since he's reborn), has to step up and save the day. This could involve facing new dark wizards or uncovering ancient secrets.
She might start off as a bit of a rebel. Since she's associated with the devils, she could be seen as going against the norms of the wizarding world. This would lead to her being more independent and not caring so much about what others think.