Another theme is self - discovery. Through their relationship, both James and Regulus learn more about themselves. They might discover new aspects of their personalities or hidden desires. In some fanfics, this is shown as they face difficult situations together and have to reevaluate who they are.
The theme of family also often comes up. Given their family backgrounds in the original story, fanfics may explore how their relationship affects their views on family. They could be seen as creating a new kind of family unit between the two of them, away from the traditional family expectations that they've had to deal with before.
One common theme is redemption. Since Regulus was involved with the Death Eaters and then had a change of heart, many fanfics use his relationship with the OC to explore his journey towards redemption. For example, the OC might be the one who helps him see the error of his ways.
One common theme is the exploration of family bonds. Since James is Harry's father, many fanfics focus on how they could have interacted if James had been alive. Another theme is the passing on of knowledge or skills from father to son.
Survival is also likely to be a theme. Given the nature of the source material, they may have to rely on each other to survive against zombies or other threats. Their relationship could be strengthened through these shared survival experiences. For example, they might have to work together to find food or shelter, and this could bring them closer together.
The exploration of his relationship with Sirius is also a common theme. Since they are brothers with very different paths, fanfics often dig into their complex relationship, from their childhood rivalry to the possibility of reconciliation or understanding in different situations.
Friendship is also a big theme. Since they are from different houses in Hogwarts, exploring how they could have become friends despite the house rivalries is interesting. Some fanfics might show them bonding over shared interests or a common goal, like fighting against the Dark Arts in a more united way than in the original story.
Friendship across generations is also quite common. Ron is from a later generation compared to Regulus. Their relationship in fanfiction can show how different experiences and knowledge from different times can merge. And often, there's a theme of discovery. Ron discovers things about Regulus that were long forgotten or buried, which then drives the plot forward.
One common theme is family loyalty and betrayal. Since they come from the Black family, their actions towards each other and the family are often explored. For example, Sirius leaving the family while Regulus had a different path within it.
Another theme is secrecy. Given Regulus' secretive nature in the Harry Potter universe, many fanfictions play on the idea of him having to keep his relationship with the character a secret from his family or other dark wizards. This adds an element of mystery and danger to the story.
Friendship is a very common theme. Their strong bond as the Marauders is often explored, showing how they rely on each other, protect one another, and have a lot of fun together. Another theme is loyalty. They are extremely loyal to each other, and this is often a central part of the stories.
Often, there might be a theme of psychological exploration. Given the complex nature of both characters, the fanfic could delve into their minds, their fears, and how they deal with each other on an emotional and psychological level. For example, how James copes with the presence of the intimidating Pyramid Head and how Pyramid Head views James, who is an outsider in his world.