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.
A rival princess from another kingdom could be the main antagonist. This rival princess might feel that Sakura is unworthy of her position and wants to claim the throne for herself. She would use her own charm, political connections, and perhaps some underhanded tactics to try and remove Sakura from power.
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.
She would likely still be strong - both physically and mentally. Her determination as a ninja would translate into her being a resolute princess, not easily swayed by others. For example, she would stand up for what she believes is right for her kingdom.
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.
Well, perhaps in the fanfiction, Sakura Haruno is suddenly transported to a magical realm where she is a princess. She meets all sorts of strange and wonderful creatures. She uses her medical ninjutsu skills to help the sick in the kingdom and gains the love and respect of the people. But there is a dark wizard who wants to steal her power, and she has to fight against him with the help of her new friends in this realm.
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.
I don't know for sure, but since it's a fanfiction centered around Sakura Haruno, she is the main focus. However, it may also feature characters who had a major impact on her life. This could include her sensei, like Tsunade, who trained her in medical ninjutsu. There's also a possibility that new characters created specifically for this fanfiction could be main characters as well, but that's just speculation.
In some Sakura Haruno alpha fanfiction, Kakashi could be a significant character. He might act as a mentor to Sakura in her new alpha role. He could guide her in making difficult decisions, like when she has to choose between the safety of the village and the life of a comrade.
One idea could be that Sakura, as a princess, has to use her medical ninjutsu skills to save her kingdom from a mysterious plague. Another could be her having to navigate through political intrigue in the court while also trying to maintain her relationships with her friends. Maybe she discovers a hidden power within herself that is related to her princess lineage and uses it to protect her people from an invading force.
Sakura might meet the founders of the village. For example, she could meet Hashirama Senju. He was a powerful and visionary leader. Meeting him could give Sakura insights into the true origins of the village and how to better protect it. She could also learn some of the ancient ninja techniques from him.
To develop Sakura in such a fanfiction, you can focus on her emotional growth. For instance, if she has a difficult beginning like being an orphan or facing discrimination, show how she overcomes these emotional hurdles. You can also develop her skills differently. If she starts training in a non - traditional way, like learning from a self - taught ninja, she might develop unorthodox fighting styles. This would not only make her more interesting as a character but also change how others in the Naruto universe perceive her.