The main antagonists could be the same organizations they have always fought against. For example, Umbrella Corporation or any other bio - terrorist groups. These groups might see Ada's pregnancy as a weakness and try to use it against them. They could target Ada to get to Leon or try to harm the unborn child to stop Leon from interfering with their evil plans.
One possible storyline could be that Leon and Ada find out about the pregnancy unexpectedly. They then have to deal with the challenges of their dangerous line of work while also preparing for a new life. For example, Leon might be extra protective of Ada during their missions.
One common theme is the struggle between love and duty. Leon and Ada are often on different sides in the official stories, so fanfics explore how their feelings for each other play out against their respective missions. For example, in many fanfics, Ada has to make a choice between helping Leon and fulfilling her own tasks.
Leon would likely become more protective. His tough - guy exterior might soften as he focuses on taking care of Ada and the unborn baby. He'd be more cautious in his actions, thinking about the safety of his family first.
In a fanfiction like this, the plot could involve Ada being pregnant with a child that has some special abilities due to their unique genetic make - up. Leon and Ada then have to figure out how to keep the child safe from those who would want to use the baby for their own purposes. It could also explore their relationship growing deeper as they face this new challenge together.
Hunnigan could be a great addition. She could provide Leon with information and support from the outside, and her presence could add some tension as she might be suspicious of Ada's motives. Another good character is Wesker. His past connection to Ada could create more drama and conflict in the story. He could be a shadowy figure trying to manipulate the situation between Leon and Ada.
Love in the face of danger is also a prevalent theme. Their jobs put them in constant peril, but that doesn't stop the development of their relationship. Fanfics often show how they find love and connection despite all the chaos and threats around them. It's a contrast that makes for very engaging reading.
Good character development makes a great Ada Wong and Leon fanfic. Writers need to stay true to their personalities. Leon's sense of justice and Ada's mysterious allure should be well - presented. Also, an exciting plot is key. Whether it's a new bio - horror threat or a complex political conspiracy, it should keep readers on the edge of their seats.