A theme that is often explored is the exploration of the characters' pasts. Since Tom Riddle jr has a very troubled past, fanfictions might dig into it more deeply. And for Harry, his connection to Tom through the Horcruxes also plays a role. By exploring their pasts, the fanfictions try to show how they could have a different relationship than what was shown in the original books.
One possible plotline could be that Harry, having seen the destruction Tom Riddle caused as Voldemort, decides to take young Tom under his wing to prevent him from going down that dark path. He could introduce Tom to the love and friendship he never had, like the friendships Harry formed at Hogwarts. This new environment and positive influence might change Tom's view of the world.
Harry would likely use his own experiences of facing inner darkness. He could talk to Tom about his struggles with the part of Voldemort that was in him. By sharing his own story, he hopes to make Tom understand that he has a choice and doesn't have to give in to any dark desires.
One interesting fanfiction could be one where Barty Crouch Jr is forced to team up with Harry Potter against Tom Riddle's resurging threat. Maybe in a scenario where Voldemort (Tom Riddle) has found a new way to gain power and Barty Crouch Jr, who has a complex history with the Death Eaters, has to side with Harry to save the wizarding world.
Well, perhaps in this fanfiction, Harry adopts Tom Riddle Jr after a time - travel incident. Harry realizes that by changing Tom's upbringing, he can prevent the rise of Voldemort. He takes Tom under his wing and shows him the value of family. Tom, growing up with Harry, starts to develop different values. He becomes interested in protecting the wizarding world rather than ruling it. This could lead to a completely different wizarding world where the threat of Voldemort never really existed.
One great 'tom riddle jr x harry potter' fanfiction is 'The Redemption of Tom Riddle'. It explores an alternate universe where Harry tries to prevent Tom from becoming Voldemort. It has really interesting character development and a unique take on the relationship between the two.
At first, it would probably be really tense. Tom is used to being on his own and having his own dark thoughts. Harry would be super cautious. But as time goes on, Harry's kindness might start to break through Tom's barriers. They could become like brothers, with Harry teaching Tom about the good in the world and Tom starting to trust Harry.
At first, it would probably be very tense. Tom is a proud and intelligent boy, and he might not take kindly to being adopted. But as Harry and Hermione show him kindness and understanding, he could start to warm up to them. For example, Hermione could introduce him to books that expand his view of the wizarding world in a positive way, and Harry could share his own experiences of growing up without parents, making Tom feel less alone.
One great 'Harry Potter Tom Riddle Fanfiction' is 'The Heir of Slytherin's Redemption'. It shows a different side of Tom Riddle, exploring what could have happened if he had been given a chance at a different path. It has a really interesting plot that keeps you hooked from start to finish.
One popular idea in such fanfiction could be that Harry, through some magical time - turning or alternate reality scenario, discovers Tom Riddle before he fully becomes Voldemort. Harry might see the good in Tom that was lost over time and tries to prevent his fall to the dark side. Their soulmate connection could be a powerful force that draws them together, even as their destinies seem so opposed.