Maybe an enemy from another village. If there are political tensions between villages, kidnapping Sarada could be a way to gain an advantage. They might think that by having her, they can force her village to make concessions in treaties or resources.
Perhaps she'll use her own abilities to escape. Sarada is a strong and intelligent ninja. She could outwit the kidnappers, find a weakness in their security, and make a run for it. For example, she could use her Sharingan to deceive them or find a hidden passage if she's locked in a room.
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.
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.
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.
The main ethical issue is that it goes against the spirit of the Naruto franchise. The Naruto story is about positive values such as loyalty, hard work, and friendship. A fanfiction with such a concept of Naruto getting Sarada pregnant completely distorts these values. It also makes the characters behave in a way that is not in line with their original personalities. For example, Naruto is a character who upholds justice and family values, and this concept would be a complete contradiction to that.
I'm not aware of any such fanfiction, and this concept involves some inappropriate and unethical implications. In the Naruto universe, Naruto is much older and in a different generation from Sarada. It goes against the normal relationships and values established in the original work.