Redemption is a common theme. Since Tom Riddle later becomes Voldemort, many fanfictions with an oc adopting him focus on whether he can be redeemed through the love and care of the oc. Another theme is the power of family. The oc often tries to create a family - like environment for Tom, which is something he lacks in the original story.
There's also 'Adopting the Dark Lord'. In this fanfiction, the oc is from a different magical family and takes Tom under their wing. The story delves deep into the Hogwarts era of Tom Riddle and how the oc's influence might change his future. It has some great character development and a lot of plot twists.
I really liked 'My Adopted Son, Tom Riddle'. This fanfiction has a unique take on the idea. The reader not only adopts Tom but also tries to shield him from the dark influences at Hogwarts. It has some great plot twists and the writing style is very immersive.
Perhaps in the fanfiction, Harry adopts Tom Riddle when he's still a young boy at the orphanage. Harry then uses his own experiences of growing up without parents to bond with Tom. He teaches Tom about magic in a different way than Dumbledore did, focusing on the positive aspects of magic for good. This could lead to Tom not becoming the evil Voldemort we know.
One great 'Tom Riddle and OC Fanfiction' is 'A Darker Shade of Magic'. In this story, the OC is a powerful witch who enters Hogwarts during Tom's time. She challenges his ideas and there are a lot of intense magical duels between them. It's really exciting to see how their relationship develops from mutual suspicion to a strange kind of understanding.
The story could also involve Hermione using her magical knowledge to help Tom control his powers. Since Tom is a powerful wizard from a young age, he might have difficulty handling his magic properly. Hermione, being a brilliant witch, could guide him. This could lead to a unique bond between them, where Tom respects Hermione for her abilities and Hermione sees the potential for good in Tom.
At first, it might be rocky. Tom would be suspicious of Harry's intentions. But as Harry shows him genuine care, Tom would start to open up. Eventually, they could become like family, with Tom looking up to Harry as a role model.
One possible plot twist could be that Tom Riddle's dark tendencies start to resurface despite Hermione's positive influence. Maybe he begins to secretly manipulate those around him at Hogwarts while still maintaining a facade of being a reformed child. This could lead to a rift between Hermione and some of her friends who suspect Tom but don't understand why Hermione is so trusting.
One possibility is that Harry, having seen the dark path Tom Riddle took, decides to take him in early. He might teach Tom about love and friendship, things he lacked growing up. Harry could introduce Tom to the wizarding world in a more positive light, showing him the joys of Hogwarts and the value of good relationships, thus preventing him from turning into Voldemort.
One common theme is redemption. Since Tom Riddle later becomes Voldemort, the idea of fem Harry being able to change his path and redeem him is often explored. It gives a sense of hope that even the darkest of characters can be saved.