It could be that kaldur believes Nightwing has some information that is crucial for a greater good, but he has to take extreme measures to get it. So, he kidnaps Nightwing. However, Nightwing might not be aware of what kaldur is after and they end up in a conflicted situation where they have to communicate to resolve the misunderstanding.
Maybe kaldur has been brainwashed. He might be forced to kidnap Nightwing by some evil force. The story could then focus on the other heroes trying to rescue Nightwing and break kaldur's mind control.
Perhaps kaldur is being blackmailed. If he doesn't kidnap Nightwing, something bad will happen to someone he cares about. Once he has Nightwing, the two of them could work together to figure out who is behind the blackmail and stop the real threat.
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.
In such a fanfiction, the plot might be centered around Red Hood having some sort of vendetta against Nightwing. Maybe he believes Nightwing has wronged him in the past, and the kidnapping could be a way to get revenge or force Nightwing to face something. For example, Red Hood could be trying to expose a secret that Nightwing has been hiding.
One possible motive could be revenge. Maybe Nightwing has thwarted Slade's plans in the past, and Slade wants to get back at him. Another could be that Slade sees Nightwing as a valuable asset. He might plan to use Nightwing's skills for his own evil purposes, like forcing him to steal something or gather information for him.
It could be someone manipulating Red Hood. For instance, a powerful crime lord who has some dirt on Red Hood and forces him to kidnap Nightwing. So, in a sense, this crime lord would be the real villain.
Well, perhaps in this fanfiction, there's a multiverse rift that pulls Nightwing into the Avengers' world. The Avengers initially mistake him for an enemy, but after some misunderstandings are cleared up, they realize his skills. Together, they face a new threat created by a combination of Hydra and some of Nightwing's rogues gallery who have also crossed over. Nightwing could teach the Avengers some stealth tactics while they show him how to handle more powerful cosmic threats.
One possible plot could be that Nightwing is somehow involved in a case that crosses over into the NCIS jurisdiction. Maybe there's a criminal organization dealing with both high - tech gadgets (Nightwing's area of expertise) and military - related crimes (NCIS). The NCIS team initially suspects Nightwing but then realizes he's an ally and they team up to take down the bad guys.
Well, one possible plot could be that a villain casts a spell on Nightwing. His friends then have to take care of baby Nightwing while trying to find a way to reverse the spell. They might discover new and cute things about him, like his love for a particular toy or food.
He may also try to talk his way out. Nightwing is known for his charm and intelligence. He could try to reason with Slade, find some common ground, or play on Slade's emotions. Maybe he could remind Slade of a past event where they had a moment of mutual respect, hoping to make Slade let him go.
One possibility is that Artemis has been misinformed. Some enemy of Percy has tricked her into thinking that Percy is a danger to the demigods. So she kidnaps him to stop him from causing harm. But as she keeps Percy captive, she slowly realizes that she has been deceived when Percy starts to explain his side of the story and the real threats he has been fighting against.
The plot could involve the Phantom seeing Christine as his only salvation from his lonely and tortured existence. So, he kidnaps her with the intention of making her understand his world. He may show her his musical creations, hoping she'll come to love him in return. However, Christine might be initially terrified and try to escape.