A really interesting plot could be that Sonic is kidnapped into another dimension by a powerful being. In this dimension, the laws of physics are different, and Sonic has to figure out how to use his speed in new ways to escape. His friends back in his own dimension are desperately trying to find a way to bring him back. Meanwhile, Sonic meets some strange new allies in this other dimension who help him in his quest to return home.
One possible plot could be that the kidnappers are actually fans who have gone too far in their obsession. They take Tom Holland, thinking they can have him all to themselves. Another could be that he's kidnapped as part of a ransom plot by a criminal group who wants a large sum of money from his management. Maybe it's also a case where a supervillain from one of his movie universes kidnaps him for some strange, other - worldly reason.
One possible plot could be that Charlie is kidnapped by a group who wants to use his mathematical genius for their own illegal purposes, like cracking codes for a major heist. Another could be that a deranged fan kidnaps him because they believe Charlie has some sort of secret knowledge that can change the world. Maybe it's a rival mathematician who is jealous of Charlie's fame and kidnaps him to try and steal his research ideas.
Well, a plot could be that Percy is kidnapped by a sorcerer who has a vendetta against Poseidon. The sorcerer thinks that by taking Percy, he can get revenge on the sea god. Another could be that some jealous demigods from another camp, who feel overshadowed by Percy's fame, kidnap him. They might try to make him look bad in front of the other demigods. Also, it could be that a mysterious force from the Underworld kidnaps Percy because they fear his growing power and want to keep him out of the way.
One common plot could be Sonic trying to rescue Chris from the kidnappers. Sonic's super speed would be a huge advantage in tracking down the kidnappers' location quickly. He might use his Sonic Spin Dash to break through any barriers in his way.
Well, in a 'Sonic gets kidnapped by Eggman' fanfiction, there could be a plot where Eggman tries to turn Sonic into a weapon. He might experiment on Sonic, but Sonic resists with his strong will. Meanwhile, Sonic's friends like Tails start searching for him. Tails could use his intelligence to track down Eggman's hideout. And when they finally find Sonic, there's an epic battle between Sonic's friends and Eggman's forces to free Sonic.
His friends like Tails and Knuckles are likely to try and save Sonic. Tails is smart and could come up with a clever plan, while Knuckles has the strength to fight off Scourge if needed.
It could be Dr. Eggman. He's always after Sonic and would love to get his hands on him to stop Sonic from foiling his evil plans. He might want to study Sonic's speed and use it for his own machines.
There could also be a group of villains working together to kidnap Sonic. For instance, a combination of Eggman and some of his more powerful robot creations along with a couple of other villains from Sonic's rogues gallery. They might have a grand plan that requires Sonic's capture, like using his power to open a portal to a world full of chaos energy that they can then control.
Well, in this kind of fanfiction, Sonic might be held in a strange and dangerous place. His friends like Tails would surely start a rescue mission. They could search all over the Sonic's world, facing various obstacles on the way. Maybe the kidnapper has some evil plan related to Sonic's speed or his Chaos Emeralds.
Well, a very intriguing plot is when Edward Nygma tries to solve a mystery that has personal implications for him. For example, in some fanfictions, he might be trying to figure out his own past or family secrets. Another interesting plot is when he teams up with unlikely allies. This could be someone from the police force or another villain. It makes for some really exciting interactions. Also, the plot where he is forced to confront his own insanity can be quite fascinating.