One possible plot twist could be the way he, now she, interacts with the original male characters. For example, the dynamic with Ron and Hermione might change completely. Instead of a male - male - female friendship triangle, it could turn into a more complex female - male - female relationship. She might also face different challenges in Hogwarts, like dealing with gender - specific expectations in the wizarding world.
In fanfiction where Harry is reborn as a girl, a major plot twist could be her relationship with Snape. Snape might have a different attitude towards her right from the start. Also, her magical abilities could be perceived differently in the wizarding society. She may be more or less expected to conform to certain female roles in magic. And the way she forms alliances within Hogwarts could be completely different, for instance, she might be more likely to befriend the female characters first and form a strong all - female group that has a big impact on the overall story.
One possible plot twist could be that as a girl, Harry has an even stronger connection to magic related to femininity, like a special bond with female magical creatures. Another could be that she faces different social challenges in the wizarding world, perhaps more pressure to conform to traditional female roles while still being a hero.
One common plot twist could be that as a reborn fem Harry as a Black, she discovers new magical abilities that were suppressed in her previous life. For example, she might have a stronger connection to nature - based magic which is more prominent in some African - inspired magical traditions within the wizarding world. Another plot twist could be her facing different forms of discrimination, not just because she's a woman now but also because of her new racial identity. This could lead to her forging new alliances with characters who are also marginalized. And she might also find out that her family has a hidden past related to the Black community in the wizarding world, which changes her perception of her place in it.
Well, it could be that Harry, as James' brother, discovers that there were more secrets about his first life than he thought. Maybe he finds out that there was a hidden power within him that was never fully awakened in his previous life. This new discovery could change the way he approaches magic and his role in the fight against the Dark Arts, and it might also lead to a different ending to the battle against Voldemort.
In this fanfiction, the plot could involve Naruto's new life as a girl leading to different relationships. Maybe she forms closer bonds with some female characters who can now relate to her on a different level. Also, her old friends like Sasuke and Sakura might have mixed reactions. Sasuke could be initially confused by this change, while Sakura might become more protective or competitive in a new way.
One possibility is that he would use his prior knowledge to prevent the tragedies that befell his family. He could also have a different dynamic with Sirius and Remus, as he'd be part of James' inner circle from the start.
A major plot twist might be that an old enemy from either the Harry Potter universe or the Naruto universe targets their relationship. For example, Voldemort could see Ino as a threat to his plans and try to separate them. Or an Akatsuki member could be interested in the power of the wizarding world and use Ino to get to it. Also, there could be a twist where Ino loses her ninja abilities in the wizarding world and has to rely solely on Harry, which changes their dynamic.
One possibility could be that he would bring his magical skills into the world of Westeros. He might use magic secretly to gain an edge in battles, like creating distractions or protecting himself and his allies. For example, he could use a shielding charm during a skirmish with the wildlings.
One interesting plot could be Harry being reborn as a Stark and using his magical knowledge in the world of Westeros. He could secretly use magic to protect his family from the various threats like the Lannisters or the White Walkers. Maybe he uses a spell to enhance the strength of the Stark soldiers' weapons.
In such a fanfiction, the plot could involve the brother having memories of a past life. This would lead to him guiding Harry in ways that Dumbledore or others couldn't. Maybe they'd explore new areas of the wizarding world that were previously unknown, all while trying to keep their relationship a secret from Voldemort and his followers. Their bond could be a key factor in the ultimate battle between good and evil in the wizarding world.
A great plot twist could be that when she is reborn, she discovers that her family has a hidden dark secret that was covered up for generations. This changes her perception of her family and her own identity.