One possible plot could be that Sauron somehow manages to enter the wizarding world. Maybe through a powerful dark magic spell that links the two universes. Harry, being the hero, senses the new and greater evil. He then has to figure out how to defeat Sauron, which is much more difficult than any foe he has faced before. He might have to learn about the power of the One Ring and how to destroy it, all while protecting Hogwarts and the wizarding world from Sauron's dark minions.
Perhaps the plot could start with a strange object appearing in Hogwarts that has Sauron's essence within it. It starts to corrupt the students and faculty. Harry, Hermione, and Ron notice the strange behavior and start to investigate. As they dig deeper, they realize it's linked to Sauron. They then find out that Sauron wants to use the magic in the wizarding world to regain his full power. Harry and his friends have to race against time to stop Sauron from achieving his goal. They might even have to make a journey to the land of Mordor in the wizarding world's version, which is filled with all sorts of new and terrifying creatures.
Well, one possibility could be that Harry discovers his true parentage as Sauron's son in a very shocking way. Maybe through some dark magic revelation. Then he might struggle with his identity, being a part of the wizarding world that he loves but also having this dark connection. He could face a lot of inner turmoil as he tries to decide whether to embrace his dark heritage or fight against it.
One possible plot could be that the fem Sauron, with her dark powers, infiltrates Hogwarts. She could initially be disguised as a new student or a teacher. Harry Potter senses something is off about her. There could be a series of magical duels and battles of wits between them as Harry tries to expose her true identity while she tries to carry out her own evil plans within the wizarding world.
In a fanfiction where Harry Potter fights Sauron, Harry would be at a disadvantage in terms of raw power. Sauron is an ancient and extremely powerful being. However, Harry has his courage and quick thinking. He could use his ability to solve mysteries and puzzles. He might figure out a way to destroy the One Ring, which would be a huge blow to Sauron. But it wouldn't be easy as Sauron would be constantly trying to stop him.
One possible plot could be that Harry Potter grows up in Mordor. Sauron might train Harry in dark magic, but Harry could have an internal struggle as he has an inherent sense of good from his parents. He might try to find a way to use the power he's learning for good, against Sauron's wishes.
Well, it could be that Harry somehow gets transported to Middle - earth and becomes Sauron. Maybe he has to deal with the power and the evil associated with being Sauron while still having some of his Harry Potter traits, like his sense of loyalty and courage, which could create an internal struggle.
A frequent theme is the idea of good versus evil on a grander scale. Harry Potter represents the side of good in his world, and Sauron is the ultimate evil in his. When combined, it's about seeing how the fight between good and evil plays out in a new and complex scenario. For example, the characters from Harry Potter might have to adapt their ideas of evil when faced with Sauron's level of malevolence. Another common theme could be the power struggle. Sauron is known for his thirst for power, and in the Harry Potter universe, there are also those who seek power, like Voldemort. Fanfictions may explore how Sauron's presence affects the power dynamics already in place in the wizarding world.
One potential character could be a new student who arrives at Hogwarts with a mysterious past. He shows extraordinary magical abilities that seem a bit too dark for the normal wizarding world. Another could be a distant relative of Draco Malfoy. Since the Malfoys have a history with the dark arts, it wouldn't be too far - fetched for one of them to be revealed as the heir of Sauron. Or perhaps an old wizard who has been in hiding for centuries and resurfaces as the heir of Sauron.
One idea could be that the heir of Sauron somehow infiltrates Hogwarts under a false identity. He tries to use the chaos of the wizarding world to his advantage to gather power. Another could be that there is a prophecy that links the heir of Sauron with Harry Potter, and they must either fight or unite depending on how the story unfolds. Or perhaps the heir of Sauron is actually a long - lost relative of one of the Hogwarts professors, which adds a layer of mystery and conflict.
In a 'Harry becomes Sauron' fanfiction, it might start with Harry being disillusioned with the wizarding world. He discovers some ancient magic that has a connection to Sauron's power. As he delves deeper, he becomes influenced by Sauron's will. He could then try to reshape the wizarding world in his image, which is now a dark and tyrannical one. He might even try to bring Middle - earth and the wizarding world together under his rule.
A creative idea could be a magical artifact that links the two universes. It's an object that Sauron wants to use to gain power over the wizarding world. Harry discovers it first and has to keep it safe from Sauron's forces.