In some fanfictions, a plot twist could be that the last Horcrux has the ability to defend itself in ways that Voldemort didn't even know. It might be able to create illusions to deceive Harry and his friends, leading them on a wild goose chase until they figure out how to break through the illusions and reach the Horcrux.
One interesting plot twist could be that the last Horcrux is not an object but a person no one suspected. For example, it could be a seemingly innocent bystander who has been under Voldemort's control all along without anyone realizing.
One of the most interesting characters could be an original character created by the fanfiction author. For instance, a wizard who has been secretly studying Horcruxes for years and has unique knowledge that helps Harry. He could be an outcast from the wizarding community but has a heart of gold.
One interesting plot twist could be that Draco has some hidden knowledge about the Horcruxes from his family's dark secrets. Maybe his mother had unknowingly passed on some crucial information to him that helps Harry and him in a very unexpected way during the hunt.
One interesting concept is the idea of creating new Horcruxes. In some fanfictions, a character might attempt to create a Horcrux in a different way than Voldemort did, perhaps using a more unique object or spell. Another concept could be the destruction of Horcruxes in unexpected ways. For example, a fanfic might explore if a Horcrux could be destroyed by a powerful emotion rather than a specific magical weapon.
One interesting plot twist could be that Harry discovers he has the ability to communicate with the Basilisk on a deeper level than just Parseltongue. Maybe he can see through the Basilisk's eyes and use this to his advantage in uncovering secrets within Hogwarts that even Dumbledore doesn't know. Another could be that the Basilisk is not truly a villain but was being controlled by a dark force all along, and Harry has to find a way to break that control and free the Basilisk.
One interesting plot twist could be that the basilisk has an unexpected alliance. Maybe it teams up with a group of outcast magical creatures to take revenge on those who have wronged them. This would completely change the dynamic as in the original, the basilisk is mainly a tool of Voldemort.
In this fanfiction, a great plot twist might be that the gamer's knowledge of game strategies is used to outwit Voldemort in a completely unexpected way. Instead of a traditional magical duel, the gamer uses their understanding of leveling up and collecting power - ups to somehow weaken Voldemort's magic. Maybe they find a way to 'hack' into his dark magic, using skills from the game world that no one in the wizarding world had thought of before.
Well, in some 'Harry Potter and the Prophecy' fanfiction, a plot twist could be that the prophecy was not a prophecy at all but a spell cast by an ancient wizard to set in motion a series of events for a greater purpose. For example, it was to bring about a new era of magic where Muggle - borns and pure - bloods could co - exist peacefully. This would completely change the way the characters view the prophecy and their roles in it. Also, the prophecy might be tied to a different character than Harry or Voldemort, like a minor character who suddenly becomes central to the whole story.
One interesting plot twist could be that instead of Ron leaving and coming back, Hermione actually leaves the group for a while because she discovers a dark secret about her family's past that she needs to figure out on her own. This would add a whole new layer of drama to the story.
Another concept could be centered around the characters' reactions when they finally locate the seventh horcrux. There could be internal struggles among the characters, like Harry having doubts about whether destroying it is the right thing to do, considering it might hold some knowledge that could be useful against Voldemort.
One idea could be that the character discovers a hidden Horcrux in a place no one expected, like in an old, abandoned wizarding prison. They have to face numerous dark magic traps alone. Another could be that they are being hunted themselves while on the Horcrux hunt, adding an extra layer of danger.