It could be the person who is jealous of Sakura. This character might try to undermine Sakura's seduction missions out of envy. For example, they could spread false rumors about Sakura to make her targets distrust her.
Maybe the main antagonist is someone who has a vendetta against Sakura's organization. They see Sakura's seduction missions as a threat and use the jealousy within Sakura's team to their advantage. They could manipulate the jealous colleague to betray Sakura or interfere with her missions in more subtle ways, like stealing important information she needs for her seduction tasks.
Well, perhaps in this fanfic, Sakura is a spy on these seduction missions. The jealousy part could come in when her partner or a friend starts to feel left out or envious of the relationships Sakura is building during her missions. For example, Sakura might be getting closer to a powerful figure as part of her mission, and her teammate, who has feelings for her, becomes jealous of the time she spends with that figure.
Maybe the main antagonist is an evil organization that has been secretly infiltrating the village. They target Sakura because of her skills and knowledge. They deceive her into believing that her village is the real enemy, causing her to betray her comrades.
One possible plot could be that Sasuke sees Sakura getting close to another male character. He starts to feel jealous but doesn't know how to handle it. So he just watches her from afar, his eyes filled with a mix of envy and longing.
Another possibility is a power - hungry ninja within the village who wants to take advantage of Sakura's situation. They might try to manipulate her or use her as a pawn in their scheme to gain more power. Since she's been abandoned, she may be more desperate and easier to influence, making her a prime target for this type of antagonist.
It could be a rogue ninja who has a vendetta against Naruto. Since Naruto is now married to Sakura, this antagonist might target Sakura as well to get to Naruto. Maybe this rogue ninja was once defeated by Naruto and now wants to take revenge by causing harm to his new family.
Itachi himself could be seen as the main antagonist in this scenario. After all, he is the one doing the kidnapping.
There could be a group of mean - spirited fairies. They see Sakura as an intruder in their territory, which might be a beautiful garden that Sakura often visits. To drive her away, they use their fairy magic to turn her into a cat, thinking that she will be less of a threat in that form. And they keep thwarting her attempts to turn back into a human throughout the fanfic.
Perhaps a new, powerful and mysterious ninja who has a vendetta against the Hidden Leaf Village where Sakura is from. He sees Zetsu as an accomplice of the village and targets both of them, causing chaos and danger in their lives. This new character could bring in a fresh and unexpected element of danger to the fanfic.
It could be a rogue ninja who has a vendetta against Naruto and wants to target his family. Maybe this ninja was once defeated by Naruto and now wants to take revenge by harming his loved ones, especially his and Sakura's child.
Maybe it's a rival within the mafia. Someone who sees Sakura as a threat to their own position or power. This person could constantly try to undermine her and make her life in the mafia even more difficult, even if the mafia boss doesn't have any ill intentions towards her at first.