Another possibility is that it's a group of mercenaries hired by a powerful underworld figure. The figure might want Nightwing out of the way so they can carry out their illegal activities more easily. The mercenaries would be well - trained and equipped, making it a difficult situation for Nightwing to escape from.
One possible scenario could be that Nightwing is kidnapped by a new supervillain who wants to use him as bait to lure Batman. The villain might hold Nightwing in an abandoned warehouse, setting up various traps. Another could be that a criminal organization kidnaps him to extract information about the other heroes in Gotham. They might use high - tech devices to try and break his will.
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 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.
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.
In many fanfictions where Gil gets kidnapped, it could be for various reasons. Maybe it's by a rival group who wants to use him as a bargaining chip against his friends. For example, in some stories, Uma's crew might be targeted because they are seen as a threat in the Isle of the Lost. Gil, being a part of that crew, gets kidnapped. Writers often use this plot device to create drama and show the loyalty and courage of the other characters as they try to rescue him.
It's also possible that Nightwing and Slade end up having a long conversation during the captivity. Slade might try to convince Nightwing to join his side, perhaps by appealing to some sense of justice that he thinks Nightwing is missing. Nightwing, of course, would resist, but this could lead to some interesting character development for both of them as they debate their different ideologies.
In the fanfiction, Nightwing could be used by the Joker as bait to lure Batman into a trap. The Joker might have some elaborate and sadistic plan in store for both of them. Nightwing, being the strong - willed character he is, will likely try to resist the Joker's psychological games and attempts to break him while waiting for rescue.
One idea could be that Sam is kidnapped by a group of rogue hunters who think he's gone too far in his use of demon powers. They lock him in an abandoned bunker and try to 'exorcise' the 'demon' out of him, not realizing he's in control. His brother Dean has to race against time to find him, following a series of cryptic clues left by the kidnappers.
Natsu was walking through the forest one day when suddenly a group of dark mages emerged from the shadows. They quickly overpowered him with some sort of magic -binding spell. They dragged him away to an old, creepy lair. His friends back at the guild soon realized he was missing and set out on a search mission. Happy flew around frantically looking for any sign of his friend. Lucy, Gray and Erza teamed up and followed some leads they found in the forest. They were determined to rescue Natsu no matter what it took.