Alaric could be developed by showing his growth in terms of his relationships with other characters aside from Caroline. Maybe he mends some broken friendships or makes new allies. His skills as a teacher could also be enhanced in the fanfic. As for Caroline, she could be further developed by having her explore different aspects of her vampire nature. She could learn new powers or ways to control her bloodlust better. Their characters could develop together by going through a series of challenges that force them to rely on each other's strengths and overcome their weaknesses.
For Alaric, his character could be developed further by exploring his past traumas more deeply. Maybe he has some unresolved issues from his time as a vampire hunter that resurface when he's around Caroline. For Caroline, her character could grow by having her face more complex moral dilemmas. For example, if she has to choose between helping Alaric or following the rules of the vampire community. They could also develop their relationship by having them share more personal stories and secrets with each other.
First, study their characters from the original show. Know their personalities, likes, and dislikes. Then, come up with a unique plot idea. It could be a new adventure, a mystery they solve together, or a relationship - building story. For example, you could set it in a different time period.
For Boruto, he could be developed by facing more complex moral dilemmas. For example, having to choose between the safety of the village and the life of a friend who has been misled. Mitsuki, on the other hand, could be developed by exploring his origin more deeply. Maybe he discovers more about his 'parent' Orochimaru and how that affects his identity as a ninja.
Bart's character could be developed by making him more responsible. In the fanfic, Greta could be the catalyst for this change. She could give him tasks to do that are important, like helping her with a community project. As he does these things, he realizes he can be more than just a troublemaker. Greta, on the other hand, could be developed by showing her flaws. Maybe she's overly perfectionist and Bart helps her see that it's okay to make mistakes.
For Ardyn, dig deeper into his emotions. Show his vulnerability when it comes to Aera. Perhaps write about his dreams and nightmares related to her. In terms of Aera, develop her character by showing her influence on the people around her. You can also give her a more active role in the events that unfold, rather than just being a passive love interest. Let her be the one who tries to change Ardyn's fate actively.
Character development could occur through their interactions with other characters. If they meet new androids or humans, how they react can show different sides of their personalities. For instance, if they encounter a human who is prejudiced against androids, RK800 might try to reason with the person, while RK900 could be more defensive. This would show RK800's more diplomatic side and RK900's protective nature.
You can try some fanfiction websites like Archive of Our Own (AO3). There are a large number of fanfics about Caroline and Alaric on it. Just search for their names in the search bar, and you'll get a list of relevant stories.
Romance can also be a theme, although it might be a more complex and nuanced kind of romance given their relationship in the original story. Some fanfics explore the possibility of a deeper romantic connection between them, delving into their feelings and how they might navigate a relationship in the face of various obstacles and social expectations.
One great 'Alaric and Caroline Fanfic' is 'The Unlikely Bond'. It explores their relationship in a unique way, with a plot full of mystery and friendship development. Another good one could be 'A New Chapter for Alaric and Caroline' which delves into how they support each other through tough times in a fictional post - show scenario.
For Flora, you could explore her insecurities more deeply. Maybe she doubts her ability to be a strong partner to Helia in dangerous situations. This could lead to her pushing herself to grow and become more confident.
To develop 2D further, focus on his growth as an artist. Show him struggling with writer's block and then finding new inspiration in unexpected places. This could be through meeting a new type of person or experiencing a different culture. For Noodle, you could develop her character by exploring her family background. Maybe she discovers she has a long - lost relative who is also a great musician or artist, and this new connection changes her perspective on her own identity within the Gorillaz.