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, 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.
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, 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.
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.
Perhaps in a fanfiction, an enemy uses death magic to bring back powerful villains from Naruto's past. Naruto then has to use his own skills and newfound knowledge of death magic (maybe learned from a wise hermit) to defeat them all over again. This could lead to some really intense battles and character development for Naruto as he grapples with this new threat.
One possible plot could be Regulus coming back and trying to right the wrongs he left behind. He might join the fight against the remaining dark forces, using his knowledge of the Death Eaters' secrets. Another could be his reunion with Sirius, leading to a complex emotional journey as they deal with their past issues.
Well, in some fanfictions, Voldemort might plug into Harry's memories. He could find out about Harry's early years at the Dursleys and use that knowledge to manipulate him. For example, he could threaten to expose Harry's Muggle relatives to the wizarding world in a negative light if Harry doesn't cooperate with him. It could also be that Voldemort plugs into Harry's magic, trying to siphon it off for his own purposes, which would lead to a power struggle between them within Harry's own magical core.
A possible plot could be a spell that makes Hermione fall in love with the wrong person. This would create a lot of drama as she struggles with her new - found feelings while her real friends try to show her the truth and break the spell. It could also explore themes of manipulation and the power of true friendship.