There's also 'Ada and Leon: Shadows of Raccoon City'. This fanfic takes place after the events in Raccoon City. It shows how their paths cross again and how they deal with the lingering traumas from that time while still being drawn to each other. It's a really engaging read as it adds new layers to their relationship.
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.
You can start by looking on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just type in 'Ada Wong and Leon' in the search bar, and a whole list of relevant fanfictions will come up. AO3 has a great tagging system which makes it easier to find specific types of stories, like ones focused on their relationship or action - packed adventures they might have together.
One possible plotline could be Ada and Leon teaming up against a new bio - terrorist threat. They might start off distrusting each other as in their previous encounters, but gradually learn to rely on one another. Another could be a story set in a post - apocalyptic world where they are the only two left with the skills to fight off the mutated creatures. Maybe there's a love story woven in between their dangerous missions, where they struggle with their feelings for each other while trying to survive.
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.
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.
One common theme is trust. Since they are often from different backgrounds or have different goals, building trust between Ada Wong and Welker is a key aspect in many fanfictions.
There's also 'Alliance in the Dark'. In this fanfic, Ada and Claire are forced to work together in a very dangerous situation. The author does an excellent job of building tension throughout the story. Their relationship is portrayed as complex, with both trust and suspicion playing a role. It's a thrilling read.