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.
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 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 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.
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 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.
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.
A good old fanfiction could be the one where Tom Riddle's relationship with the Chamber of Secrets is explored in more detail. It might show how he found out about it and his early experiments with the Basilisk. This could give a new perspective on the events in the original books.
A possible plot could be that Tom Riddle is forced to save Harry Potter as part of a magical pact or curse. He initially resents it but as he spends time with Harry, he starts to see the world through Harry's eyes. For example, Harry's kindness towards others, even those who are different or outcasts, could make Riddle question his own beliefs. The fanfic could then focus on how Riddle tries to balance his old dark desires with his new - found respect for Harry and the values Harry represents. There could be a lot of internal conflict within Riddle as he tries to decide whether to fully embrace this new path or return to his old ways.
It allows for new storylines. For example, in some fanfictions, a female Harry Potter might be able to understand Tom Riddle's past in a way that the male Harry couldn't. This could lead to attempts at redemption or different forms of conflict. There are also fanfictions that explore a romantic relationship between them, which is a huge departure from the canon and thus very interesting.
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.