First, think about what would make Donatello turn 'dark'. It could be a betrayal, a lost experiment, or some external influence. Then, develop his character in a way that shows his change. For example, his speech might become more cynical. Next, create a conflict that his new 'dark' self has to face. This could be against his old friends or a new enemy.
A 'dark Donatello' fanfiction might explore the idea of him being an outcast among his own kind. Maybe he discovers a dark secret about his origin that makes him question his place in the world. This could lead to him becoming bitter and angry, and his actions would be different from the usual heroic Donatello. For example, he might start to isolate himself and work on projects that are considered unethical by his family.
One possible analysis could be that their relationship in fanfiction often explores a deep bond. Maybe it delves into how their different personalities, Donatello's intelligence and Raphael's brashness, interact and complement each other. It could also show their growth as characters through their relationship, like learning to understand and support one another better.
A possible plot twist could be that April is pregnant, but it turns out that there was some sort of mix - up or false positive. This would cause a great deal of emotional turmoil for both of them. They would have to deal with the disappointment and also re - evaluate their relationship and future plans.
One idea could be that they face challenges in a world that is not fully accepting of their relationship. They might have to deal with the reactions of their friends and family. For example, some of their friends could be initially shocked but then gradually come to support them. The fanfiction could focus on how they grow together as a couple during the pregnancy, learning about each other's fears and hopes.
Love and attraction is another common theme. Writers like to explore the budding romance between them. It could be a slow - burn kind of love where they gradually realize their feelings for each other. Sometimes it's shown through their small gestures and glances.
One of the common themes is mutual respect. In fanfiction, they often come to respect each other's skills and abilities. Also, the theme of loyalty is prevalent. They stand by each other through tough times. For example, when one of them is in danger, the other will go to great lengths to save them. Additionally, the theme of brotherhood is often explored, highlighting their connection as part of the same team.
One interesting storyline could be them going on a joint adventure to solve a mystery that threatens their world. They might combine their unique skills, Donatello's intelligence and Leonardo's leadership, to outwit the villains.
Splinter is another character that could be included. As their mentor, he can offer wisdom and guidance to Donatello and Leonardo. His influence can shape their relationship and how they deal with the challenges in the fanfiction.
One popular theme is the 'opposites attract' concept. Donatello, being a tech - savvy and often introverted turtle, and April, a more outgoing human, create an interesting dynamic. Another theme could be the idea of friendship evolving into something more. As they spend time together fighting villains and solving mysteries, their bond deepens in fanfictions.