There could be internal antagonists within Naruto's own village. Some of the more conservative ninja in the village might not trust Naruto's new - found skills and powers obtained from Zetsu. They could try to undermine Naruto or even attempt to exile him, fearing that he has been corrupted by Zetsu's teachings.
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.
Naruto could be the one to save her. He has a strong bond with Sakura and would never let her be in danger for long. His powerful ninja skills and his unwavering determination would make him a likely candidate to rescue her from Black Zetsu's grip.
Well, one possibility could be that Zetsu imparts some unique and dark jutsu to Naruto. Maybe Naruto starts to learn stealth and subterfuge skills from Zetsu, which are different from what he learned at the ninja academy. This could lead to Naruto having an edge in certain battles where traditional ninja skills aren't enough.
One possible plot could be that Zetsu, with his unique abilities, somehow crosses paths with Sakura. Maybe he's on a mission related to the Akatsuki and she gets in his way. He could initially view her as just another obstacle, but as time goes on, he realizes her strength and tenacity. This could lead to an unlikely alliance where they work together to achieve a common goal, like saving a village from a greater threat.
Zetsu could be sarcastic towards Sakura, taunting her about her human emotions. Sakura, in turn, would stand up for herself and show her strength. For example, Zetsu might say something like 'Your soft heart will be your downfall, little girl' and Sakura would reply 'My heart is what makes me strong, something you could never understand.'
Black Zetsu's story in Naruto is quite the tale. He was an entity that had been around since ancient times. He had an in - depth knowledge of the ninja world's history and used this to his advantage. He was the driving force behind many of the conflicts in the Naruto series. For example, he caused a rift between Sasuke and Naruto at times. His true nature was only revealed towards the end of the series, which was a shock to many characters. He was a master of manipulation, using the desires and weaknesses of others to achieve his ultimate aim of bringing back Kaguya.
One key event was his deception of Madara. He made Madara think he was his ally while actually setting him up for Kaguya's revival. Another was his influence on Obito. He pushed Obito to do things that served his purpose. Also, his role in the Fourth Great Ninja War was significant as he was behind many of the unexpected turns in the war.