Well, it could be one of the rivals of the empress or someone with a hidden agenda. Maybe it's related to power struggles or personal vendettas. But it really depends on the specific plot and characters of the manga.
The Great Tang Almost Abducted Empress Zhangsun at the Beginning of the Game was a novel titled " The Great Tang: Almost Abducted Empress Zhangsun at the Beginning of the Game!" This novel told the story of a male singer named Lin Xuan during the Tang Dynasty. Lin Xuan was the only male singer in the brothel of Chang 'an City. His handsome appearance and talented artistic talent attracted the worship of countless noble ladies and young ladies. However, he rejected everyone's pursuit, claiming that he only wanted to live off a woman quietly. Even so, he was appointed as the Minister of Works, which caused dissatisfaction and confusion among the civil and military officials. This novel showed the social style and character relationships of the Great Tang Dynasty through Lin Xuan's experience.
The name of the empress in One Piece was Boya Hancock. Boya Hancock was a character in the Japanese manga One Piece and its derivative works. She was the only female in the Seven Kings Sea of Martial Arts, the captain of the Nine Snakes Pirates, and the highest leader of the Amazon Yuri fighting race, the Nine Snakes. She was regarded as the world's number one peerless beauty, with special abilities and domineering colors. She had a story with the protagonist Monchi D. Luffy in the plot and helped him successfully reach the city. As for the empress in other works or characters, the search results did not mention any relevant information.
Sam is also a probable candidate. Given her feisty nature, it could lead to some interesting scenarios where she tries to fight back against the kidnapper. And her relationship with Carly and Freddie would drive them to take action.
Papyrus can also be a victim of kidnapping in fanfictions. His naive and friendly nature makes him an easy target in some stories. His kidnapping could lead to Sans going on a desperate search for him, which would involve a lot of action and emotional moments as Sans tries to rescue his brother.