Her super strength could be a great asset. For example, she could physically protect Harry from attacks that he might not be able to fend off on his own. Say, when Death Eaters ambush them, she can easily throw them off.
The female vampire's immortality could bring a different perspective to the story. She might have memories or knowledge from long ago that could be crucial in defeating Voldemort. And her ability to heal quickly could mean she can take more risks in battles, leading to more daring rescue attempts for Harry and his friends.
The female vampire could introduce a new element of danger. If she is not fully in control of her bloodlust, it could put the Hogwarts students at risk. This would force Harry to not only deal with the external threats like Voldemort but also manage the internal situation with the vampire. On the other hand, if she can control it, her blood - related powers, like the ability to heal herself quickly, could be a great asset in battles.
It could be a new character created specifically for the fanfiction. Maybe a vampire who has been living in the Forbidden Forest for years and has stayed hidden until some event brings it to Harry's attention.
One great fem Harry Potter vampire fanfiction is 'The Vampire's Apprentice'. It takes the idea of Harry being a girl and adds the vampire element in an interesting way, with a lot of action and a unique take on the magical world.
It could be an entirely new character created just for the fanfiction. Maybe a long - lost wizard who was turned into a vampire and then gained enough power to become a king among them.
Harry's magical abilities would be adapted to the new environment. His knowledge of defensive magic would be crucial. He could use the Shield Charm in new ways, like protecting against vampire compulsion. His flying skills on a broomstick might not be as useful as in Hogwarts, but he could learn to use magic to levitate himself quickly to avoid vampire attacks. Also, his ability to sense danger, which he developed from his experiences in Hogwarts, would be heightened in this more dangerous Vampire Diaries world.
One common theme is the struggle between the magical world of Harry Potter and the vampire world. For example, in many fics, female Harry has to deal with the different rules and powers in both worlds.
One common theme is the transformation. The process of Harry becoming a female vampire and how it affects her physically and mentally. For instance, the new senses and cravings she has to deal with.
I really liked 'A Potter in the Night Class'. In this fanfic, the fem Harry Potter is transported to the Vampire Knight universe. The author does a great job of integrating the two universes. For example, Harry's magic interacts with the vampire powers in unexpected ways. It also has a great deal of character development as she tries to fit into this new world and deal with the different factions, like the Day Class students and the vampires of the Night Class.
Well, the story might progress as Harry struggles to control his new vampire instincts while still fighting against the dark forces in the wizarding world. He could form an alliance with some vampires who are on the good side, if there are any. This would give him access to new powers and knowledge. Meanwhile, Voldemort or other dark wizards might try to use his new condition against him. For example, they could try to frame him for attacks on other wizards so that the wizarding community turns against him.
The magic system from Harry Potter could potentially nullify some of the vampire abilities in Twilight. For example, a simple shielding charm could prevent a vampire's super strength from breaking through. On the other hand, vampires' enhanced speed might make it difficult for a wizard to cast spells accurately. It would be a constant battle of who could outmaneuver the other. Also, the use of potions in Harry Potter could have an impact. A potion that makes a wizard invisible might be very useful against vampires who rely on their sight.