Well, if Naruto is captured by Orochimaru in fanfiction, one possibility is that Orochimaru wants to use Naruto as a bargaining chip against Konoha. He may think that by having Naruto in his grasp, he can force the Hidden Leaf to meet his demands, like giving him access to certain forbidden jutsu or secret scrolls. Another aspect could be Orochimaru's curiosity about Naruto's ability to master new jutsu so quickly. He might want to study Naruto's mind and body to find out the secret behind his growth as a ninja.
If Naruto gets captured by Orochimaru in fanfiction, his friends' reactions would be intense. Hinata, who has deep feelings for Naruto, would be heartbroken but also very determined. She might try to secretly track Orochimaru on her own at first, using her Byakugan to search for Naruto. Kiba with Akamaru would be eager to jump into action, using their tracking skills. Lee would be excited to prove his strength and save Naruto, ready to use his Taijutsu skills against Orochimaru. His friends' combined efforts would be focused on rescuing Naruto from Orochimaru's clutches as soon as possible.
It's possible that some unexpected characters could come to Naruto's rescue. For example, an old enemy of the Akatsuki who has a change of heart. Or perhaps some of the Jinchuriki who Naruto has befriended in the past. They might feel indebted to Naruto for his kindness and decide to help him out of the Akatsuki's clutches. There could also be a mysterious and powerful hermit ninja who hears about Naruto's capture and decides to intervene for the greater good of the ninja world.
In this fanfic, Naruto might be tortured for information about the Hidden Leaf. The Akatsuki are known for their ruthless ways, so they could use various jutsu to make him talk.
In this fanfiction, Luffy might be used as a bargaining chip by his captors. Maybe they'll try to exchange him for something valuable, like a powerful Devil Fruit or a large amount of treasure. His crew would then surely launch a rescue mission, facing many challenges along the way.
One common theme is the rescue mission. The friends of Tails will usually band together to save him. For example, Sonic will use his speed to find out where Tails is held and then they will all figure out a plan to break him out.
In this fanfiction, perhaps Serena is captured by an evil organization. They might be after her special powers or knowledge. Then, her friends would start a rescue mission.
Maybe it's an old foe who has managed to regain power and wants to make Sora pay for past defeats. For example, a powerful dark sorcerer who was once defeated by Sora but has now found a way to capture him through some devious plot.
Well, in such a fanfiction, Sora might be held in a dark and gloomy dungeon. Maybe some old enemies from the original story could be behind the capture, seeking revenge.