One common theme is friendship. They are both part of the Supergirl team, so many fanfics explore their bond as friends, how they support each other during tough times.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of fanfics on it. Just search for 'Alex Danvers and Winn Schott' in the search bar, and it will show you relevant works.
I'd recommend 'The Danvers Sisters' Journey'. It focuses on their growth as individuals and as sisters. It has some really heartwarming moments, like when they have late - night talks and share their deepest fears. It also shows how they deal with the villains in their own unique ways, all while strengthening their relationship.
One possible fanfic could focus on a mission gone wrong that leads to both Alex and Kara experiencing PTSD. In the story, they might initially try to hide their symptoms from each other. Kara could be having nightmares about not being able to save the people she loves during the mission, while Alex might be jumpy and overly vigilant. As the story progresses, they finally open up to each other and start to heal together, perhaps through some shared training sessions that also double as therapy, where they learn to trust themselves and each other again.
A good 'Alex Danvers and Kara Danvers Fanfic' has to capture their unique relationship. Since they are sisters, it should show the love, bickering, and support that comes with that. For example, if it has a scene where Alex is training Kara and is tough on her but also encourages her, that's great.
One possible theme could be family relationships. Since Alex is Winn's mom, it might explore how that family dynamic works, like the love, care, and sometimes the challenges that come with it.
Another theme is the idea of finding one's place. Ruby, being a young character, is often on a journey to figure out where she belongs in the world. And Alex, with her own experiences, helps Ruby in this quest. In some fanfics, this is shown through Ruby joining Alex in her superhero activities and realizing that's where she wants to be.
One possible fanfic could focus on how they support each other through their PTSD episodes. Alex, with her military background, might have PTSD from combat situations. Kara, having lost her planet and family, also has her own traumas. They could share their nightmares and fears, and use their superpowers in non - physical ways to comfort each other, like Kara using her heat vision to create a warm and calming environment when Alex is having a flashback.