In certain fanfictions, a really interesting plot twist is when Harry Potter is sent back in time and ends up influencing Tom Riddle's early life in ways he didn't anticipate. This can lead to a completely different Hogwarts and wizarding world. For example, Harry might accidentally introduce a new form of magic that Tom then misuses. Or, there could be a plot twist where Tom Riddle's Horcruxes have an entirely different origin or purpose than in the original books, which throws Harry's understanding of how to defeat him into chaos.
One plot twist could be that the time travel is not accidental. In a fanfic I read, Dumbledore sent Harry back in time on purpose to try and redeem Tom Riddle. This changes the whole perspective of the story as it shows Dumbledore's more proactive and perhaps riskier side in dealing with the threat of Voldemort.
One interesting plot twist could be that Hermione's teachings accidentally lead Tom Riddle to an even darker path. Maybe she teaches him advanced magic that he then uses in unexpected and malevolent ways. For example, he could use time - turning magic she showed him to manipulate historical events to his advantage.
Tom Riddle in fanfic can also be a central character in alternate universes. He might be redeemed in some stories, which is a really interesting take. This could involve him having different experiences that lead him to question his dark desires and ultimately turn away from the path of evil. It gives a whole new perspective on his character.
One possible plot twist could be that Tom Riddle kidnaps Harry not for the typical reasons like revenge or to gain power over the wizarding world. Maybe he discovers that Harry has some hidden knowledge about a long - lost magic that he wants for himself. Another twist could be that while kidnapped, Harry manages to find a way to get into Tom Riddle's mind and discovers his deepest fears, which he then uses to his advantage to try and escape. It could also be that there are other dark forces at play that Tom Riddle is actually trying to protect Harry from, in his own twisted way, by taking him away from the wizarding world for a while.
In some fanfictions, like 'The Unforeseen Consequences of Time Travel', a plot twist could be that when Harry and Hermione go back in time to save someone, they end up causing more harm than good. This completely flips the expected outcome on its head and makes for a very engaging read.
A really interesting plot twist would be if Harry Potter was somehow transported to Gotham as a child and raised there. He would grow up with a different set of values and skills, and when he discovers his magical abilities later on, he has to balance his newfound magic with the street - smart tactics he learned in Gotham. This would completely change his character and how he approaches the fight against Voldemort. And when Batman eventually meets him, he's not the typical Harry Potter we know from the books. It could lead to a whole new dynamic between them.
One interesting plot twist could be that Harry goes back in time and accidentally changes the sorting of the houses. This would lead to different friendships and rivalries. For example, if he made it so that Draco was sorted into Gryffindor, it would completely change their dynamic. Another could be that he changes the outcome of a Quidditch match in the past, which then has a ripple effect on the school's morale and subsequent events. Maybe Gryffindor loses a crucial match they were supposed to win, and it makes them more determined in other aspects.
One possible plot twist could be that Tom Riddle discovers Harry has a hidden power that he didn't expect. Maybe Harry has the ability to resist Riddle's mind - control spells in a unique way. This could lead to a battle of wills between them where Harry tries to outwit Riddle while being held captive.
One great Tom Riddle fanfiction is 'The Heir of Slytherin's Redemption'. It delves into the idea of Tom Riddle having a change of heart. Another is 'Tom Riddle: The Lost Years' which explores what he might have been up to during the time not fully covered in the original books. And 'Riddle's Unraveling' is also good as it shows different perspectives on his fall to the dark side.
One interesting plot twist could be when a potion that was meant to be a simple healing draught accidentally turns into a truth - revealing potion. This might lead to characters revealing secrets they never intended to. Another could be a potion that causes temporary magical ability swaps among the characters, like Blaise and another student, which would create a lot of chaos and new interactions.