A different plot could be that Voldemort brainwashes Harry into believing he is a Death Eater. Harry then has to navigate the world of the Dark Arts, all the while his true self is fighting to break free. This could lead to some really interesting character development as Harry has to face the dark side of magic and the people within it, and then find a way to return to his old self and defeat Voldemort once and for all.
In some fanfics, Voldemort brainwashes Harry to make him a spy within the Order of the Phoenix. Harry, under the influence, feeds false information back to Voldemort while also trying to fight the brainwashing subconsciously. This creates a lot of internal conflict for Harry as he struggles between his true self and the implanted thoughts. It also makes for a very tense and exciting story as the Order members start to suspect Harry.
One possible plot could be Voldemort brainwashing Harry to turn against his friends at Hogwarts. Harry might start acting strangely, like isolating himself from Ron and Hermione. Another plot could involve Voldemort using the brainwashed Harry to steal important magical artifacts from the school. Maybe Harry is made to believe that these artifacts are rightfully Voldemort's and that he's on a noble mission.
One interesting plot could be a powerful villain using a unique jutsu to brainwash Naruto himself. This would turn the tables as Naruto, usually the hero, starts acting against his friends. For example, he might try to stop Sasuke from leaving the village for the wrong reasons, but while brainwashed, he could be used to drive Sasuke further away.
One possible plot could be Hawkeye being brainwashed to target his own teammates. He might start by infiltrating the Avengers' base under the mind control. Another plot could involve Hawkeye being used to steal important artifacts from the Avengers, all while believing he's doing the right thing due to the brainwashing. For example, he could be sent to retrieve Thor's hammer, which would create a huge conflict within the team as they try to figure out what's wrong with him without harming him.
One way could be through the power of love. Just like in the original series, Harry's strong bonds with his friends and his memories of love could start to break through the brainwashing. For example, a memory of his parents' sacrifice might resurface and shatter the false thoughts Voldemort implanted.
One possible plot could be that Harry is captured by Voldemort and his friends have to mount a dangerous rescue mission. They might have to infiltrate Voldemort's stronghold, which could be filled with dark magic defenses.
One interesting plot could be how Harry and Voldemort initially react to the betrothal. Maybe Harry is completely shocked and disgusted at first, while Voldemort sees it as a way to further control the wizarding world through Harry. Another plot point might be how the wizarding community reacts. Some might be horrified, thinking it's a dark and twisted union, while others might see it as a potential way to end the long - standing conflict between good and evil.
Another plot could be the development of the relationship between Harry and Voldemort. At first, they would be extremely hostile towards each other. But as time passes, they might start to understand each other in unexpected ways. For example, Harry could teach Voldemort about love and friendship, concepts that Voldemort has long discarded, which could potentially change Voldemort's dark path.
One interesting plot could be that they are mates due to a powerful ancient magic that was long forgotten. This magic forces them to understand each other's pasts and they start to question their long - held enmity. For example, they might discover that they both had difficult childhoods which led them to their current paths, and this newfound understanding makes them consider a different future together.
In some of these fanfictions, the plot might involve Harry and Voldemort having a secret family connection. For example, they could be distant cousins through some convoluted wizarding family tree. This changes how they view each other and the conflict between them.
In some stories, their friendship could start from a time - travel accident. Harry gets sent back in time and ends up befriending a young Voldemort. He tries to influence Voldemort's path, and they form an unlikely bond. Harry might introduce Voldemort to different values and ideas that were not present in Voldemort's original upbringing, which causes a major shift in the future of the wizarding world.
Perhaps in the fanfiction, Harry gives up because he becomes disillusioned with the wizarding world. He may see all the corruption and prejudice within it and decides that Voldemort's way of ruling might not be much worse. This could lead to a story where Harry actually joins Voldemort's side, not as a slave but as an 'advisor' of sorts, trying to change Voldemort from the inside. However, this is a very dangerous and morally ambiguous path for Harry to take.