Another theme is the exploration of their changed relationship. They may have been enemies before, but in the reborn scenarios, they could become friends or at least allies. It's like in some fictions where they team up to face a greater threat that was overlooked in their previous lives. They learn to understand each other's motives better.
One popular 'Harry and Tom Reborn Fanfiction' is 'The Boy Who Lived Again'. It has an interesting plot where Harry is reborn and has to face a different set of challenges with Tom in a new light.
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.
One common theme is redemption. Since Tom Riddle becomes Voldemort, many fanfictions explore the idea of the reborn character trying to redeem themselves or change the dark path. Another theme is self - discovery. The character has to figure out how to deal with being in Tom Riddle's body and mind.
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.