Well, in 'luffy x nami family fanfic', a frequent theme is the idea of home. The Straw Hat ship is their home, but with the addition of a family - like relationship between Luffy and Nami, it becomes more of a family home. They create a warm and inclusive environment for everyone on board. Also, the theme of sacrifice often comes up. Just like in a real family, they are willing to sacrifice for each other's well - being. For instance, Nami might sacrifice her share of treasure to get medicine for a sick crewmate, with Luffy's full support.
One common theme is protection. Luffy is always ready to protect Nami and in a family - fanfic context, this extends to protecting their 'family' within the crew. Another theme is growth. As they form a family - like bond, they both grow as individuals. Nami becomes more confident in her decisions and Luffy learns more about the importance of relationships other than just being comrades.
Common themes in 'luffy x nami family fanfic' include unity. Since Luffy and Nami are key members of the Straw Hat crew, when they form a family - like relationship, it strengthens the unity of the whole crew. Everyone looks up to them as a sort of parental or big - brother - big - sister figure. Another theme is the sharing of values. They pass on their values such as freedom (which Luffy highly values) and fairness (something Nami is passionate about) to the other members of their 'family' on the ship. This helps in creating a harmonious and loving family - like atmosphere within the crew.