In a fanfiction, it could be a shadowy organization. They might have heard rumors about Raven's unique powers and decided to take her. This organization could be researching ways to gain superhuman abilities and believe Raven holds the key. Or it could be a deranged fan of Raven's powers. He might be obsessed with her and think that by kidnapping her, he can have her all to himself. Additionally, a vengeful spirit from Raven's past could be the kidnapper, seeking revenge for something she did long ago.
A great plot twist would be if Raven is kidnapped and taken to a place where she meets her long - lost family members, but they're the ones who kidnapped her because they need her help to fight against a common enemy, and they knew this was the only way to get her attention. Another plot twist could be that the kidnapper has a split personality, and one side is good and wants to free Raven, while the other side is evil and wants to keep her captive. And the last plot twist might be that Raven is kidnapped into a parallel universe where everything is different, and she has to find a way back while also dealing with the kidnapper in this strange new world.
Enemies from her past, such as old villains she's fought against, would be likely kidnappers. They might hold a grudge for her previous defeats and want to use her for revenge or some dark purpose.
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 could be the shooter. Maybe the shooter has a change of heart after shooting the raven and tries to make amends. He could then become the central figure as he embarks on a journey to save the raven's life.
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.
Peter's kidnapping was a carefully planned event. The kidnappers had been observing him for weeks. One day, as he entered a park, they grabbed him. They blindfolded him and took him to a secluded location. Peter was scared but he didn't show it. He started to befriend one of the kidnappers, a young guy who seemed a bit hesitant about the whole thing. Peter told him stories about his life and gradually the kidnapper started to have second thoughts. Eventually, the kidnapper helped Peter escape by distracting the others.
It could be Beast Boy. Their relationship in the original series has some chemistry, and in the fanfiction, it would be an interesting twist if he were the father. Maybe their relationship takes a new turn when Raven discovers she's pregnant.
Well, it could be that the raven is a special or magical raven. Maybe it was flying over a restricted area and got shot by a hunter who didn't realize its importance. Then, the story could follow how the raven's friends or family try to save it or seek revenge on the shooter.