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.
It could also be an external force, like another criminal organization. They might want to get their hands on Sakura Haruno for their own purposes, and this could put her in the middle of a conflict between the mafia she is sold to and this other group. This external organization could be the main antagonist as they constantly try to kidnap or harm Sakura.
The plot might involve Sakura Haruno being betrayed by someone close and then sold to the mafia. Once in the hands of the mafia, she could be forced to do various tasks against her will, like spying on other clans or stealing valuable information. She might also try to find a way to escape while dealing with the dangerous and often immoral members of the mafia.
It could be an envious noble. This noble might be jealous of Sakura's position as a princess and her popularity among the people. So, they try to spread false rumors about her and plot to dethrone her.
In the fanfiction, the main antagonist could be a dark and ancient force that has awakened. Sakura fakes her death to get closer to this force without it suspecting her. She then tries to find a way to defeat it, while the force believes it has eliminated one of its potential threats.
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.
Another possible antagonist could be a jealous relative. For example, a cousin who believes they should be the heir instead of Sakura. This relative could spread false rumors about her, or try to sabotage her efforts to prove her royalty in more subtle ways, like stealing important documents or artifacts related to her claim to the throne.
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.