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.
Ardyn's character can be developed by looking at his relationship with power. How did he view power before he was corrupted? And how does his love for Aera play into that? You can also explore his sense of betrayal, not just by the gods but also by the people he once cared about. For Aera, you can expand on her backstory. What made her the person she is? What were her hopes and dreams before she got involved with Ardyn? By exploring these aspects, both characters can be made more complex and interesting in the fanfic.
One plot idea could be exploring their relationship before the fall. Maybe a story of how they met and the initial attraction. Another could be a what - if scenario where Aera manages to save Ardyn from his dark path. Or perhaps a fanfic set in an alternate universe where they are able to live a normal life together, away from the chaos and tragedy of their original story.
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.
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.
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.
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.
For Booth, you can focus on his military background more when they are undercover. Maybe he has some special combat or survival skills that come in handy in a dangerous undercover situation. This can also show his loyalty and protectiveness towards Bones in a new light.
For Jessie, you could develop her character by showing her dealing with a fear she has. Maybe she's afraid of heights but has to face it during an adventure with Lemon. This would add depth to her already adventurous character. As for Lemon, you could explore her family background. If she comes from a long line of Disney performers, it could influence her actions and decisions in the fanfic.
For Ichigo, his character could be developed by exploring his inner turmoil more deeply. Maybe he struggles with the idea of being half - human, half - Shinigami in a different way. As for Rukia, she could be developed by showing more of her past, like her early days in the Soul Society before she met Ichigo. This would give more depth to her character.