Another option is that it could be some sort of ancient race or tribe that has a grudge against pirates. They capture Luffy thinking he's a typical pirate causing chaos, but then realize he's different as they get to know him during his captivity.
It could be the Marines. They've been after Luffy for a long time, so it's possible they finally manage to capture him in this fanfiction.
It could be the League of Villains. They've always been interested in taking down heroes and Deku, as a rising hero, would be a prime target for them.
Maybe a group of rogue Pokémon hunters. These are people who don't follow the normal rules of Pokémon catching. They might be after Darkri because it's a rare and powerful Legendary Pokémon. They could sell Darkri on the black market or use its powers for their own gain, like using its dream - related abilities to find hidden treasures that are only revealed in dreams.
A group of outcast ninja who have been wronged by the Hidden Leaf Village in the past might be the captors. They see Naruto as a symbol of the village that wronged them and Itachi as a powerful Uchiha. Capturing them would be their way of getting revenge on the village.
There's a possibility that it's a villainous trainer. This trainer might capture Gengar with the intention of using it for evil purposes, like using Gengar's powers to steal other Pokémon or cause chaos in Pokémon battles.
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.
The Thanagarians could be the captors. They are an alien race with advanced technology. If they perceive Wonder Woman as a threat to their interests in Earth or the universe, they might capture her. Also, an ancient, long - forgotten race of demons could rise from the underworld and capture her. They might want to use her as a key to unlock some sort of dark power that has been sealed away for centuries. This would create a really interesting and spooky fanfiction scenario.
The captors could be Orochimaru. He is always interested in powerful ninja and their abilities. Since female Naruto would be a very powerful individual, he might capture her to study her abilities or try to extract the Nine - Tails' chakra from her. Another possibility could be a group of anti - Konoha rebels who see Naruto as a symbol of Konoha's strength and want to weaken the village by capturing her.
The Hidden Stone Village might be the captors in some fanfictions. If there was a political struggle or a misunderstanding, they could see Naruto as a weapon of Konoha that needs to be neutralized. Their earth - style jutsus could be used to create inescapable prisons for Naruto, and they might use various interrogation techniques as torture to find out Konoha's secrets.
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.
Maybe a rogue ninja group that wants to sell Naruto to the highest bidder. They see him as a valuable asset due to his strength and status in Konoha.