A plot could involve a spell gone wrong in the Harry Potter world that brings Eren Jaeger into the story. He might initially be seen as a threat because of his different nature. But then, he teams up with Harry Potter and his friends. Eren could teach them some of his combat tactics from his world, while they help him understand magic. Together, they face challenges like dark wizards who are also interested in Eren's powers for their own evil purposes.
In this fanfiction concept, perhaps the magic in the Harry Potter universe is connected to the power of the Titans in some way. Eren Jaeger could be on a quest to find out how his world and the wizarding world are related. He might discover that his Titan - shifting ability has roots in an ancient magic that was lost in the wizarding world. Along the way, he forms relationships with the characters in Harry Potter, and they all have to work together to stop a greater evil that threatens both of their worlds. This could involve battles, magical research, and lots of character development as they learn from each other's cultures and abilities.
I'm not sure if it's extremely popular. Fanfiction often combines different universes in unique ways, but this particular combination might be a niche idea. It depends on the overlap between Harry Potter and Attack on Titan fandoms.
Another idea is that their cousin relationship causes some drama at Hogwarts. Thorus might be in a different house than Fem Harry Potter, and there could be a rivalry between the houses that affects their relationship. Or perhaps Thorus gets jealous of all the attention Harry gets for being 'the chosen one' in this female version, which could lead to some tense moments.
In a 'harry potter is danny phantom fanfiction', we could imagine that the wizarding world has a hidden connection to the ghost zone. Harry, being the hero, discovers this connection. He teams up with some of Danny's friends or allies from the Danny Phantom world. Together, they try to close a portal that is allowing dark forces from both worlds to merge. Along the way, Harry has to learn how to control his newfound ghostly powers, which are different from his magic. Hermione and Ron are initially skeptical but soon realize the importance of these new powers in the fight against the combined evil of Voldemort and the ghost - related villains from Danny's world.
One popular 'female Eren Jaeger fanfiction' is 'The Female Titan's Secret'. It explores an alternate storyline where Eren is a female and has different relationships with other characters. Another great one could be 'Female Eren: A New Perspective' which delves into how her being female changes the overall narrative of the original story.
One possible plotline could be that Ainz Ooal Gown somehow gets transported into the Harry Potter universe. He might use his powerful magic and strategic mind from his own world to deal with the threats in the Harry Potter world, like Voldemort. Maybe he would try to take over Hogwarts or form an alliance with Dumbledore. Another could be that Harry Potter gains some of Ainz's powers, like his ability to control undead, which would completely change the dynamic of his fight against Voldemort.
One common theme is identity. Since Eren is female in these fanfictions, there's often exploration of how she fits into the world that was originally designed around a male Eren. Another theme is relationships, like her friendships or potential romances with other characters in a different light.
One popular female Eren Jaeger fanfic is 'Eren's Redemption'. It explores a different side of Eren's character through the eyes of a female protagonist. The story delves into his internal struggles and how the female character helps him face his demons.
Perhaps in the fanfiction, Mikasa believes that Eren has gone too far with his plans and she betrays him to stop him from causing more destruction. She might think that his actions are endangering not only their friends but also the entire world. So, she makes the difficult decision to betray him in an attempt to save everyone else.
It could be that the female Harry Potter travels to the world of Nara and discovers new forms of magic there. Maybe she has to adapt to the different magical customs and make new friends or allies.
The plot could start with Harry having strange visions or experiences that lead him to discover his connection to the White Lantern. As he delves deeper into this new power, he realizes that it can be used not just for fighting dark wizards but also for restoring the balance of life in the wizarding world. He might use it to heal the damage done by the Dark Arts, revive extinct magical creatures, and bring hope back to the wizarding community. His friends might be initially confused or even a bit scared of his new powers, but they soon come to understand and support him as he uses the White Lantern powers for good in the Harry Potter universe.