A plot twist might be that the goblins turn against Harry midway through his visit. They could be influenced by a dark wizard who has a grudge against Harry. This would completely change the course of his visit and force him to find a new way to achieve his goals in Gringotts. Another possibility is that the vault contains a magical item that has a mind of its own and tries to possess Harry when he touches it.
In this fanfic, Harry might encounter new magical creatures in Gringotts. Maybe he discovers a secret vault that was thought to be lost.
One possible plot twist could be that the inheritance is cursed. Instead of being a boon, it brings trouble to fem Harry. For example, every time she tries to access a part of the inheritance, she is plagued by dark visions or dangerous creatures.
A plot twist might be that when Harry goes to Voldemort, he discovers that Voldemort has a secret weakness. Maybe it's related to his family or some long - forgotten magic. Harry then has to decide whether to use this weakness against Voldemort or try to redeem him. This would add a whole new layer of complexity to the story. Another aspect could be that there are other forces at work, like a third - party wizard who is manipulating both Harry and Voldemort for their own gain, and Harry has to figure out who this is while dealing with Voldemort.
In fanfic, when Harry Potter goes to Gringotts, he might discover some long - lost secrets about his family's vault. There could be hidden spells or artifacts that were left there by his parents or ancestors.
Perhaps a plot twist in this fanfic could be that Bella, while living with Billy and Jacob, starts to have strange dreams that seem to be prophetic. These dreams could lead her to uncover a mystery that has been buried for a long time in the Quileute tribe. Or, there could be a sudden arrival of an old enemy of Billy's, who now targets Bella because of her connection to them, forcing Jacob to protect her in a more dangerous and complex situation than ever before.
Perhaps a major plot twist could be that Voldemort actually has a change of heart after 'adapting' Harry. He sees something in Harry that makes him question his own evil ways. This could lead to a strange alliance between the two where they try to find a new way to rule the wizarding world that is less about destruction and more about a new form of magic governance. It would be a very unexpected turn considering Voldemort's character in the original series.
Maybe Harry has to compete in some magical challenges set by Gringotts to retrieve a powerful magical item. For example, he might face enchanted vault guardians.
A really interesting plot twist could be that the school has a secret passage or a hidden room that is somehow related to the demigod world. Percy and Annabeth accidentally discover it while exploring the school. Inside, they find ancient artifacts or prophecies that lead them on a new and dangerous adventure, which might involve traveling to a different dimension or time period.
One possible plot twist could be that the girl who is pregnant has some sort of magical ability related to the pregnancy that no one expected. Maybe she can communicate with the unborn child in a magical way. Another could be that Harry discovers he's not the real father, but due to some magical mix - up, he has to take on the role anyway.
A possible plot twist in a 'James takes Harry' magic fanfic could be that James is not exactly as Harry imagined him. Maybe he has some secrets or a different personality than what Harry has been led to believe through stories. For example, James could have had some dealings with dark magic in the past that he has to come clean about to Harry.