Her friends could also play a crucial role. If she has a group of loyal friends, they might band together to find her. They could use their knowledge of the area where she was last seen or any clues they can gather to track down the kidnappers and save her.
In the Naruto world, Naruto is Himawari's father. Incestuous relationships like the one described in the title are not acceptable in the context of the story's values. Fanfiction is supposed to be a creative expansion of the original story within the bounds of its themes and ethics. So it's highly unlikely there would be any proper fanfiction with this idea.
There shouldn't be any such fanfiction. Boruto and Himawari are siblings in the Naruto universe, and creating a story about them in such a way goes against the basic values of family and propriety. It would also be unacceptable to a large number of fans who respect 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.
One possible motive could be ransom. If Himawari is from a well - off family, the kidnappers might be after a large sum of money. Another motive could be revenge against someone close to Himawari, like her parents. Maybe they have some sort of vendetta and think kidnapping her will be a way to get back at them.
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.