Harry might develop a stronger sense of justice due to his Gryffindor influence, while Tom could be more influenced by Slytherin's ambition. So Harry focuses on helping others, and Tom on achieving his own goals.
Tom, on the other hand, could start to see the value in friendship and loyalty through working with Harry. His character could soften a bit, realizing that there are other ways to gain power and respect than through fear and intimidation. For example, he could learn from Harry's ability to inspire people. They could also have a sort of mentor - mentee relationship in some aspects, with each teaching the other different spells or magical theories. This would make Tom a more complex and well - rounded character.
They could start by having small moments of understanding. For example, when one of them is in trouble, the other shows unexpected kindness. This could gradually build trust between them.
There's also 'Harry and Tom: The Forbidden Alliance'. It's a fanfiction that delves into a rather complex relationship between the two. They find themselves in a situation where they have to form an alliance against a common, dark enemy, and the story is filled with intense magical battles and emotional turmoil.
I think Luna Lovegood would be an interesting character to include. Her unique perspective on magic and the world around her could add an element of mystery and wonder to the fanfiction. Also, Severus Snape. His complex past and relationship with both Harry and Tom (in a way, as Tom became Voldemort) could bring a lot of depth to the story. And Hagrid, who could provide a connection to the more gentle and nurturing side of the wizarding world, which might be important for Harry and Tom's relationship development.
Hermione could be a major character. She might use her intelligence to try and figure out where Tom has taken Harry. She could be researching old spells or looking through Tom's past hideouts to find clues.
It's hard to name the best ones as it's a very subjective matter. However, you can start by looking at the most reviewed and bookmarked stories on popular fanfiction websites. The authors of those stories are likely to be very good at writing Harry and Tom fanfiction. Also, ask in fan communities. Other fans might be able to recommend some great authors based on their own reading experiences.
In this type of fanfiction, the magic systems associated with both characters can be a major focus. Harry's magic is often more instinctive and influenced by his emotions and the need to protect. Tom Riddle's magic, especially as he delved deeper into the Dark Arts, has a more sinister and calculated feel. Writers may create scenarios where the two magics clash or, interestingly, where they have to work together. This can create some really exciting magical battles or collaborations within the story.
Another theme is redemption. Some fanfictions like to explore if Tom could be redeemed through his relationship with Harry. It gives a new perspective on the character of Tom Riddle.
I really like 'A Twist of Fate in Hogwarts'. In this fanfiction, Harry, Draco, and Tom are all thrown into a series of mysterious events. It's fascinating to see how the author weaves their individual personalities into the plot. Tom's cunning, Draco's pride, and Harry's courage all play important roles in solving the mysteries.