Another possibility is that an old enemy of the Uzumaki clan resurfaces. They use Itachi's complex relationship with the village and his own motives to manipulate him into kidnapping fem Naruto. So, this old enemy would be the main antagonist as they are the ones driving the overall conflict.
Well, in this fanfiction, perhaps Itachi is being manipulated by some dark force. He kidnaps fem Naruto because he's under the control of this force which thinks that Naruto, being female in this version, holds the key to some sort of ancient prophecy. Itachi is then in a struggle between his loyalty to the village and the commands of this dark entity.
Itachi himself could be seen as the main antagonist in this scenario as he is the one carrying out the kidnapping. He is acting against Sakura's will and causing a dangerous situation for her.
Prussia would likely be the main antagonist in this scenario since he is the one carrying out the kidnapping. His actions are against Fem Canada's will, which makes him the opposing force in the story.
Itachi himself could be seen as the main antagonist in this scenario. After all, he is the one doing the kidnapping.
Itachi himself could be seen as the main antagonist in this scenario as he is the one performing the kidnapping. His actions are against Sakura's will and are likely to cause distress and conflict.
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.
Madara Uchiha could still be a great antagonist. Even with Itachi and Naruto teaming up, Madara's power and his long - term plans for the ninja world would pose a huge threat. His knowledge of the Sharingan and his connection to the Uchiha clan would make for an interesting conflict against Itachi, while his overall strength would be a challenge for Naruto as well.
It could be Orochimaru. He has always been interested in obtaining powerful jutsu and the Sharingan. If he catches wind of Itachi teaching Naruto, he might see Naruto as a new target to either steal power from or use as a pawn against Itachi and the Leaf Village.
It could be Orochimaru. He might have manipulated Kushina into kidnapping Naruto. Orochimaru has always been interested in the power of the Uzumaki clan and Naruto's nine - tailed fox power. He could be using Kushina as a tool to get to Naruto.
Yes, there might be such fanfictions out there in the vast world of fan - created content. However, this is a rather unique and often controversial concept within the Naruto fanfiction community.