A really interesting plot twist could be that killing Voldemort early somehow disrupts the balance of magic in the wizarding world. Maybe dark magic starts to seep out in unexpected ways and Harry has to deal with the consequences of his actions in a way he never expected. For example, spells start going haywire and he has to find a way to fix it all.
One popular 'Harry kills Voldenmort early' fanfiction is 'The Early End' on FanFiction.net. It has an interesting plot where Harry discovers a new spell that gives him the upper hand early on.
One possible plot twist could be that Dumbledore was under the influence of a dark magic spell that made him kill Hagrid. And Harry, realizing this, has to find a way to break the spell and clear Dumbledore's name while also dealing with his own grief and anger over Hagrid's death.
One possible plot development could be that Harry kills Umbridge in self - defense. Maybe she attacks him first with the intent to harm or kill, and Harry has no choice but to use lethal magic in return. This would then lead to a huge uproar in the wizarding world, with some believing Harry was right and others seeing him as a murderer.
One possible plot twist could be that while captured, Harry discovers a hidden power within himself. Maybe it's a new type of magic that he unlocks due to the extreme situation. This power allows him to not only break free from his captors but also gives him an edge over Voldemort. Another aspect could be that the capture was part of a plan all along, by Dumbledore or some other wise wizard, to deceive Voldemort and get Harry in a position where he can deliver the final blow.
A third plot twist could involve a character we least expect coming to Harry's rescue. Perhaps a Slytherin student who has been secretly against Voldemort all along. This student could have some inside knowledge about Voldemort's lair and manages to free Harry, leading to a change in the dynamic between the houses at Hogwarts.
One possible plot twist could be that the betrothal was a setup by a dark wizard to gain control over Harry. Maybe they thought by tying him to a Slytherin like Blaise, they could manipulate Harry more easily.
One possible plot twist could be that Harry has hidden magical abilities passed down from Regulus that no one expected. Maybe he can communicate with certain magical creatures in a unique way.
In such a fanfiction, it could be that Harry Potter has been corrupted by a powerful and malevolent force. This force might have warped his sense of right and wrong. It could also be that the author wants to explore the extreme consequences of power and how it can turn a hero into a villain. For example, if Harry Potter was constantly taunted and mistreated by the wizarding community despite all he has done, he might reach a breaking point and lash out at everyone.
In certain fanfictions, a plot twist is that Harry doesn't kill Voldemort directly. Instead, he uses his love and friendship magic in a very unique way. He manages to make Voldemort see the error of his ways, and Voldemort sacrifices himself to undo all the evil he has done. This is a very different take from the usual 'kill or be killed' scenario. Another interesting plot twist could be that the killing of Voldemort reveals a long - hidden prophecy about Harry that has nothing to do with the original prophecy, and this new prophecy sets Harry on a whole new path of discovery and adventure.
One possible plot twist could be that Snape has a hidden agenda all along. Maybe he's been using Harry as a pawn in a bigger scheme related to the wizarding world's power struggle. Another could be that Harry discovers a long - lost family connection to Snape that changes their dynamic completely.