Sakura would be a likely candidate. As a skilled medical ninja, she has the knowledge and ability to diagnose the poison and attempt to create an antidote.
In this 'naruto gets poisoned fanfiction', Naruto could also try to fight the poison using his own chakra. He might enter a state of inner struggle where he has to battle the effects of the poison on his body while still trying to protect the village. His Nine - Tails chakra could play a role, either helping him resist the poison or making the situation more complicated if it reacts badly with the poison.
Perhaps an enemy from a rival group. They might see Sakura as a threat to their plans, so they poison her to get her out of the way. Another possibility is a character who has been secretly harboring a grudge against her for a long time, and finally decides to take action by poisoning her.
In a 'sakura gets poisoned' fanfiction, she might first start to feel unwell, like having dizziness and weakness. Her friends could then notice the strange symptoms and start to search for a cure.
It could be Morgana. She has been known to be an antagonist and has a vendetta against Camelot and those associated with it. Poisoning Merlin would be a way to weaken Arthur's most powerful ally.
Well, he could experience intense pain and weakness. Depending on the type of poison, it might affect his connection to his father Poseidon. Maybe he'd be unable to communicate with sea creatures or draw strength from the ocean as he usually does. His friends would be in a panic. They'd probably split up, with some staying to take care of Percy while others go on a quest to find the source of the poison and a cure. Chiron at Camp Half - Blood would also be involved, providing what knowledge he has about such poisons.
If Harry Potter gets poisoned in fanfiction, it could lead to some really interesting plot twists. His magical powers might be affected. He could start having strange visions or lose some of his abilities temporarily. Dumbledore or other teachers might get involved in a big way to figure out who poisoned him. And the whole Hogwarts community could be in an uproar, with students trying to find out the truth and protect Harry from any further harm.
Well, one possibility is that Merlin's friends, like Arthur, might search desperately for a cure. They could go on a dangerous quest to find a rare herb or consult a powerful sorcerer who might know how to save him. Another could be that Merlin, while poisoned, has strange visions or dreams that give him clues about his true destiny or the future of Camelot.
It could be a team effort. Amy with her determination, Tails with his intelligence, and Knuckles with his strength could all come together. They might search different places for the cure ingredients. Amy could search in the forest for a rare flower, Tails could look in his lab for a chemical compound, and Knuckles could dig for a special mineral. All these combined might save Sonic.