Some fanfictions may include animal characters that are associated with the sakura. For example, a magical fox that guards the sakura grove or a bird that can communicate with those who possess the sakura power. These characters add an element of magic and mystery to the story.
In many fanfictions, Sakura Haruno's secret power could be enhanced medical ninjutsu skills that she has been secretly developing. Maybe she has found a way to combine her chakra control with some ancient medical techniques, giving her the ability to heal the most severe of wounds almost instantaneously.
Maybe an old hermit who has been living in seclusion near a sacred sakura grove. He has been the guardian of the sakura power of nature for years and now has to take on an apprentice to pass on his knowledge and protect the power from those who would misuse it.
Another possible plot is a conflict over the sakura secret power. Different groups or individuals might want to possess this power for their own reasons, like for political gain or to achieve immortality. The story could revolve around the main character's attempts to protect the power from falling into the wrong hands and the battles and challenges they face along the way.
Often, there is a dark lord or antagonist who is associated with the power of darkness. This character might be power - hungry and use the dark power to oppress others.
Obviously, Sakura herself is a must - include. Then, characters like Kakashi could be involved as he has Anbu experience. Naruto and Sasuke might also make appearances, maybe Naruto trying to help Sakura in her Anbu - related adventures, and Sasuke being a bit of a wildcard. Additionally, some original Anbu characters could be created, like a veteran Anbu agent who becomes Sakura's mentor in the Anbu world.
Sakura is the main focus. Alongside her, there could be a new character who she meets by chance and who somehow gets involved in her secret. This new character could be from a different place or group and has their own motives regarding Sakura's secret.
Sasuke is also frequently present. His complex relationship with Sakura can be further explored in the fanfiction. The reincarnated character could either get involved in their relationship or try to change the dynamics between them.
Obviously, Naruto is often a central character. His journey to discover and master new secret powers is a common theme. Sasuke is also frequently involved, either as a rival or an ally in the exploration of these powers.
Often, there are also antagonist characters. For instance, a dark sorcerer who wants to use the power of the sakura reincarnation for his own evil purposes. This creates conflict in the story as the protagonists must protect the sakura spirit from being exploited.
If her secret power is strong healing abilities, it could change the plot by allowing her team to take more risks in battles. They know that even if they get seriously injured, Sakura can heal them. So they might engage in more dangerous missions or battles against stronger opponents.