He could be touched. Despite his stoic exterior, Itachi may see this as a genuine and innocent gesture from Naruto, and it could warm his heart in some way. Maybe he starts to see Naruto in a new light, as someone who can bring a bit of light - heartedness into his world of shadows.
Itachi could also be amused. He might find Naruto's quirkiness endearing and the duck could be the start of a more relaxed relationship between the two. He may even start to crack jokes about the duck or incorporate it into his daily life in some small way within the fanfiction.
The duck could symbolize peace or friendship. Maybe Naruto gives it to Itachi as a sign of truce or new - found understanding between them.
Naruto might initially be shocked and angry. He has a strong sense of justice and being kidnapped goes against everything he stands for. He would likely try to fight back immediately, using his basic ninja skills and his determination. For example, he could try to use his Shadow Clone Jutsu to create distractions and escape.
Itachi would probably be very calm and composed. He might see Sarada as a symbol of the new generation of ninjas in the village. He could be gentle with her, perhaps sensing her potential and her desire to grow as a ninja.
Naruto might give Itachi some traditional Konoha food as a gift. Since Itachi has been away from the village for a long time, he might miss the taste of home.
Sasuke would likely be enraged. Naruto is his rival and also, in a way, his friend. He would immediately set out to find Naruto and confront Itachi. Sasuke has a strong sense of pride and would not let his brother get away with this.
Naruto would probably be extremely confused at first. He has always seen Itachi as a complex figure, and suddenly being forced to marry him would be a huge shock. He might feel angry that he has no say in this. Itachi, on the other hand, would likely be stoic on the surface but internally be very conflicted. He has his own burdens and this unexpected situation would add to them.
Sasuke would probably be very confused at first. He has spent so much time hating Itachi for what he did to the Uchiha clan. Seeing Naruto befriend Itachi would make him question his own long - held beliefs. He might initially be angry with Naruto for seemingly siding with his enemy.
Naruto would probably be very confused at first. He's always seen the Akatsuki as the enemy, so being taken there by Fem Itachi would be quite a shock. He might be on guard and constantly looking for a way to escape.
Some fans might be disappointed. Fanfiction often allows them to explore different scenarios with Naruto's character. If he gives it up, it's like closing a door on those creative possibilities.
I'm not sure about a specific one off - hand, but you can check out fanfiction websites like Fanfiction.net or Archive of Our Own. Just search for 'Itachi gives Naruto power' and you'll likely find a bunch of different stories. Some might focus more on the action aspect, while others might explore the emotional side of Itachi's decision to give Naruto power.