Well, Naruto's survival could be due to a combination of things. Firstly, his friends. Even though in Orochimaru's clutches, Naruto might be holding on because he has people like Sakura, Sasuke, and Kakashi who he wants to see again. Secondly, his own unique physiology. He's the host of the Nine - Tails, and that powerful chakra could be unconsciously helping him resist the harmful effects of the experiments. Also, he might be learning to adapt during the process. He's always been a quick learner, so perhaps he figures out ways to counteract what Orochimaru is doing to him.
He survives through his strong will. Naruto has an unwavering determination that refuses to be broken by Orochimaru's experiments. His inner strength and the chakra of the Nine - Tails within him might also play a role in withstanding the physical and mental torture of the experiments.
Naruto could develop a new sense of determination. He might be determined to not let Orochimaru's experiments define him. Instead, he could use what has happened as a motivation to become stronger in his own way. He may also start to study more about the type of experiments Orochimaru was doing, not to replicate them, but to find ways to counter any future threats related to that kind of science.
He would probably be very angry. Naruto has always been a free - spirited person, and being experimented on against his will would be a huge violation of his rights. He might initially try to resist physically, but as the situation gets more complex, he could start to plot his escape and revenge.
He probably resists. Naruto has always been a fighter. He would not just sit there and let them experiment on him without a fight. He might use his basic ninja skills like substitution jutsu to try and escape the experimenters.
His character could develop in a darker direction. The torture and manipulation from Orochimaru's experiments could make him question his values. However, it could also strengthen his resolve to protect his friends and the village. He may start to see the world in a more complex way, understanding that there are greater evils out there. And his newfound powers from the experiments could give him a sense of responsibility that he didn't have before.
Well, Naruto's friends would be in an uproar. Iruka, who has always been like a father figure to Naruto, would be heartbroken and would not rest until he knew Naruto was safe. The other Konoha ninja would also be affected. Neji, despite his initial coldness towards Naruto, has come to respect him. He would see Orochimaru's actions as a threat to the village and to his friend. Tenten would offer her weapon - related skills to help in any rescue attempt. And Hinata, who has deep feelings for Naruto, would be extremely distraught. She might even try to take on Orochimaru herself in a desperate attempt to save Naruto, regardless of the danger to her own life.
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?
Well, in some fanfictions, Orochimaru might use some forbidden jutsu. He could have combined his knowledge of resurrection techniques with some unique chakra sources. Maybe he found a way to reverse the death process by using the power of the snakes he has a connection with. For example, he could have used the regenerative abilities of the snakes' chakra to bring Naruto back to life.
He could have physical deformities. Orochimaru's experiments are often extreme and might cause Naruto's body to change in abnormal ways. Also, he might have mental trauma. The experience of being experimented on could leave him with psychological scars that affect his relationships and his ability to trust.
Naruto would be angry. He's not the type to just accept being kidnapped. He'd immediately start looking for ways to escape. He might try to use his ninja skills to break free from whatever restraints Orochimaru has put on him.
Well, in some versions, Naruto might use his new and improved Rasengan techniques. He could have combined it with his Sage Mode powers to overpower Orochimaru. Maybe he also uses his quick thinking and the element of surprise, since Orochimaru is always so full of his own schemes.