Love in the face of danger is a big theme. In these fanfictions, they are in the army where there are constant threats. But their love for each other remains strong. Also, growth is a theme. They grow as individuals, learning new skills and becoming more confident in the military setting, all while maintaining their relationship.
One common theme is the struggle between two worlds. Since she is part vampire and part human (or has strong ties to both), she often has to find her place. For example, in many stories, she has to deal with the secrecy of the vampire world while still wanting to be part of normal human society.
One common theme is the protection of the child. Given Edward's vampiric nature and the various threats in their world, a lot of fanfictions focus on how they keep their child safe. For example, in many stories, they build a secure haven for the child.
One common theme could be betrayal. Maybe in these stories Edward does something that makes Bella lose trust in him and thus start to hate him. For example, he might lie about something important.
A lot of these fanfictions also deal with the idea of acceptance. Bella has to accept her new state as a vampire, and the Cullen family has to accept her as one of their own. There are often challenges and conflicts related to this acceptance process.
A common theme is protection. Edward sees himself as Bella's protector, so when he punishes her, it's often because he believes she has endangered herself.
One common theme is the power imbalance. Since Edward is the boss, there's often a play on the power dynamics in their relationship. Another theme is secrecy, as they may have to keep their relationship a secret from their colleagues in the workplace.
One common theme is the exploration of their eternal love. Writers often expand on how their love endures through different challenges. Another theme is the power dynamics, especially considering Edward's vampire nature and Bella's human - turned - vampire state in some stories.
One common theme is the exploration of love triangles. Since Edward, Jasper, and Bella are all in an interesting dynamic, many fanfictions focus on how their relationships could be reimagined in different ways regarding love and attraction.
Self - discovery is also common. Bella often has to figure out who she is in relation to Edward and the vampire world she's being drawn into. Also, family plays a role. Edward's vampire family and Bella's human family add layers to the stories.
A frequent theme is the struggle for freedom. Bella may fight against the situation she's in, but also find herself falling for Edward in the process. Also, secrecy is often a theme as they might have to hide their relationship from others due to the unorthodox way they came together.