Common themes often revolve around their journey together. In Lightning and Serah lemon fanfiction, there might be themes of protection and sacrifice. Lightning's strong will to protect Serah could be intertwined with their developing romantic relationship, which might be expressed through more intimate scenes. Also, themes of self - discovery within their relationship can be present as they grow closer to each other.
One common theme could be the exploration of their deep emotional bond in a more romantic and sometimes sensual context. For example, their reunion after a long separation might be a central plot point.
The themes in such fanfiction can vary widely. However, some recurring ones include the power of their love. Their love might be depicted as something that overcomes great obstacles, whether it's the challenges within their fictional world or personal insecurities. Another theme could be the contrast between Lightning's more stoic nature and Serah's softer side, and how they complement each other in a romantic relationship. This contrast can be further explored in the context of their more intimate interactions.