One possible plotline could be Nightwing struggling to fill Batman's shoes in Gotham. He might face challenges like dealing with more dangerous villains who see him as a new target. Another could be his relationship with the other members of the Bat - family changing as he takes on the Batman mantle.
Obviously, Nightwing himself would be the main character as he's the one taking on the Batman role. Robin, if there's a younger Robin at the time in the fanfiction, would also be a key character. He might look up to Nightwing in his new role or have some doubts about him filling Batman's shoes.
One common aspect in these fanfictions is the struggle with identity. Robin has been known as Batman's sidekick for so long. Becoming Nightwing means carving out his own path. He has to deal with how his friends in Young Justice will view him. Some might be supportive right away, while others may be skeptical at first. His costume change also symbolizes a shift in his role within the team and the wider superhero community.
One possible fanfic could involve a battle against a powerful villain. Nightwing might get injured while trying to protect Gotham. Batman then has to step in not only to defeat the villain but also to take care of Nightwing. He could use his advanced medical knowledge and resources in the Batcave to nurse Nightwing back to health.
He might try to reason with kaldur. If Nightwing can make kaldur see that what he's doing is wrong or that there are better ways to achieve his goals, kaldur could let him go. Nightwing is known for his charisma and intelligence in such situations.
He might try to engage Slade in conversation. Nightwing is smart and could be looking for a way to psychologically get an edge over Slade. He could be probing for Slade's motives, hoping to find a way to turn the situation around. He might also be stalling for time, waiting for the right moment to make his move.
Nightwing is alive in the comics. He's had his share of challenges and close calls, but as of now, he continues to fight for justice and protect the people.
Batman is the most likely. He has a strong bond with Nightwing and would do anything to save him. His skills in combat and detective work would be crucial in finding and freeing Nightwing.
A possible plot could be Nightwing getting betrayed by a close friend or ally. Maybe they were framed for something they didn't do, and this led to their heartbreak. For example, if a fellow hero believed false rumors about Nightwing and turned against him, it would shatter his trust and break his heart.
Nightwing would initially be defiant. He's a highly trained fighter and detective in his own right. So, he won't cower in front of the Joker. He'll probably try to size up the situation, looking for any weaknesses in his captivity and the Joker's plan.