I'm not sure if there is such a specific fanfiction. There are a vast number of fanfictions out there, but this particular concept might be quite rare or not in line with the general tone of the 'One Piece' series. Fanfiction often explores different relationships and scenarios, but this one seems rather out - of - the - ordinary considering the nature of the characters in the original work.
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.
In many of these fanfictions, there could be a rival trainer. This rival might be jealous of the captor for getting Darkrai and try to steal it. Also, there could be a Pokémon professor - type character who gives advice on how to handle Darkrai. For example, they might know some special techniques to keep Darkrai calm while captured.
In some fanfictions, when Gengar gets captured by Ash, it might lead to some really interesting team dynamics. Ash could use Gengar's unique abilities in battles. For example, Gengar's ability to phase through walls could be used in creative ways during gym battles or against Team Rocket. Maybe Gengar would initially be a bit rebellious, but over time, it would grow to respect Ash as its trainer. Ash would also have to find new ways to train Gengar compared to his other Pokémon, since Gengar has different strengths and weaknesses.
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.
He might be interrogated by the captors. They could try to extract information about the Sith, the Empire's secrets, or his own powers. Maybe they would attempt to turn him against the Emperor, using various means like appealing to his remaining humanity or threatening him with things he values.
The wedding would probably be a huge and chaotic affair. There would be pirates from all over coming to celebrate. Luffy's friends like the Straw Hat crew would be there, of course. There would be lots of food and drink, especially meat for Luffy. The venue might be on an island they've visited before, like Sabaody Archipelago or even on the Thousand Sunny.