One common theme is redemption. Dabi, with his troubled past, often has a journey towards redemption, and Shoto can play a part in that. Another theme is family. Given their complex family relationships in the original story, fanfics often explore how they deal with family issues together.
Well, in the MHA manga, Dabi does meet his end. It's a poignant moment that has significant implications for the story's progression and the dynamics among the characters.
There's also 'The Unlikely Bond'. In this fanfic, Dabi and Shoto start off as enemies but gradually find common ground. It has some really interesting character development and plot twists that keep you hooked.
One common theme is identity struggle. Since the OC is now in Shoto's body, they have to figure out how to balance their original self with the new identity. Another theme is power exploration. They have to learn to use Shoto's quirk effectively. And also, family relationships are often explored as Shoto has a complex family situation in MHA.
Well, there's 'My Reincarnation as Shoto' which has an interesting take on the concept. The story focuses on how the OC's memories and personality change the way Shoto approaches relationships with his family and friends in the MHA universe. There are also some fanfics that are less well - known but still great, like 'Reincarnated into Shoto's World' where the OC - turned - Shoto has to deal with the villains in a unique way based on their previous life experiences.
Midoriya Izuku could also have an interesting reaction. He's always trying to understand the villains better, and Dabi is a very mysterious villain. He might study the fanfictions to look for clues or insights into Dabi's character that he could use in a potential confrontation or in his quest to become a better hero. He might be surprised by some of the creative interpretations in the fanfictions.