Well, it's possible that in the fanfiction, Naruto is experimented on as part of a desperate attempt to save the village. There could be a new threat on the horizon, and the village elders or some ninja with knowledge of forbidden techniques believe that by experimenting on Naruto, who has shown great resilience, they can find a solution. They might be trying to turn him into a weapon or a shield for the village, which would put Naruto in a really tough situation both physically and morally.
In such a fanfiction, one possible scenario could be that Naruto is experimented on for his unique chakra. Maybe Konoha scientists, in a bid to understand the power of the Nine - Tails within him, conduct various tests that are both physically and mentally taxing for Naruto.
Well, he could gain some new and unexpected powers. Maybe he discovers a way to enhance his chakra control to an extreme level, allowing him to perform jutsus with much greater efficiency and power.
Another scenario could be in a battle. Naruto might face an opponent who is much stronger than him. In his determination not to give up, he could keep fighting even when he's already injured. He might take hits that are too powerful for him at that moment, like a powerful genjutsu that makes him harm himself unknowingly while trying to break free, or getting caught in a powerful ninjutsu attack that damages his body severely.
One possible scenario could be that in the fanfiction, some evil organization captures Percy and tries to figure out his powers related to being a demigod. They might subject him to various tests like trying to measure the limits of his ability to control water, or how he can heal from injuries that would be fatal to normal humans. Another could be that they experiment on his connection to the Greek gods, perhaps trying to sever or manipulate it in some way.
One possible scenario could be that the experimenters are trying to enhance Robin's physical abilities. Maybe they use some sort of advanced technology or strange serums. Another could be that they are trying to access his memories, perhaps related to Batman's secrets or the inner workings of the Young Justice team. It could also be an experiment to test his mental resilience under extreme stress.
In 'sonic gets experimented on fanfiction', one possibility is that the experimenters are from an alien race. They might be curious about Sonic's abilities which are far beyond what they've seen in their own world. So they capture him and start running all sorts of tests on him, from physical examinations to tests on his cognitive abilities.
Perhaps Naruto starts to have a connection to Orochimaru's other experiments or creations. He might find himself in a situation where he has to either help or fight these other 'products' of Orochimaru's mad science. Also, the fanfiction could explore how Naruto's friends react to what has happened to him. Do they trust him still? Or are they afraid of what Orochimaru might have turned him into?
One possible scenario could be Naruto exploring new jutsu by experimenting on his own chakra. Maybe he tries to combine different elemental chakra within himself in unorthodox ways to create a super powerful and unique jutsu. Another could be related to his physical endurance. He might experiment with training methods that push his body to the limit, like enduring extreme pain or fasting for long periods while still trying to perform at his best in combat simulations.
The plot could take a dark turn. Naruto's friends might become worried about him and try to stop his self - experimentation. This could lead to a conflict between Naruto's desire for power and his friends' concern for his well - being.
Well, one possibility is that in fanfiction, Death Eaters who survived the war might capture Harry Potter and experiment on him to extract the power of his connection to Voldemort. They might believe that this power could be harnessed for their own evil purposes. Also, there could be fanfictions where rogue wizards from other countries experiment on Harry, seeing him as a unique specimen due to his fame and his defeat of the most powerful dark wizard in history.