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.
In many fanfictions where Gil gets kidnapped, it could be for various reasons. Maybe it's by a rival group who wants to use him as a bargaining chip against his friends. For example, in some stories, Uma's crew might be targeted because they are seen as a threat in the Isle of the Lost. Gil, being a part of that crew, gets kidnapped. Writers often use this plot device to create drama and show the loyalty and courage of the other characters as they try to rescue him.
A more emotional plot idea could be that Nami gets pregnant by someone she loves deeply, but that person is lost or in danger. She then has to go through the pregnancy alone, with the support of the Straw Hat crew. The story could focus on her growth as she becomes a mother - to - be and how she overcomes the challenges without her partner.
It could be a new character created specifically for the fanfiction. Maybe a young scientist who is working on a project related to the super - soldier serum. Since Bucky has a history with the serum, this would create an interesting dynamic.
One idea could be that Sam is kidnapped by a group of rogue hunters who think he's gone too far in his use of demon powers. They lock him in an abandoned bunker and try to 'exorcise' the 'demon' out of him, not realizing he's in control. His brother Dean has to race against time to find him, following a series of cryptic clues left by the kidnappers.
Natsu was walking through the forest one day when suddenly a group of dark mages emerged from the shadows. They quickly overpowered him with some sort of magic -binding spell. They dragged him away to an old, creepy lair. His friends back at the guild soon realized he was missing and set out on a search mission. Happy flew around frantically looking for any sign of his friend. Lucy, Gray and Erza teamed up and followed some leads they found in the forest. They were determined to rescue Natsu no matter what it took.
Peter's kidnapping was a carefully planned event. The kidnappers had been observing him for weeks. One day, as he entered a park, they grabbed him. They blindfolded him and took him to a secluded location. Peter was scared but he didn't show it. He started to befriend one of the kidnappers, a young guy who seemed a bit hesitant about the whole thing. Peter told him stories about his life and gradually the kidnapper started to have second thoughts. Eventually, the kidnapper helped Peter escape by distracting the others.
In some fanfiction scenarios, if Nami were to side with an enemy, even if it was for a seemingly good reason like trying to protect the crew in a way that Luffy doesn't understand. Luffy has a strong sense of loyalty to his crew and seeing Nami on the 'opposing side' might enrage him. He might feel betrayed and his anger would be a natural reaction.
If there was a 'luffy gets nami pregnant fanfiction', it would likely face a lot of criticism from the 'One Piece' fandom. The relationship between Luffy and Nami in the original work is more of a deep friendship and comradeship. However, if we were to consider such a fanfiction, it could explore how their crewmates would react. Maybe some would be supportive, while others might be shocked. It could also delve into how Luffy and Nami themselves would grow and change as they take on this new role of becoming parents, which is completely different from their usual pirate - hunting and treasure - seeking activities.