On Naruto's side, he would probably feel guilty after realizing what he had done. This guilt could make him try to make amends, but Sakura's initial reaction of anger and hurt would still be a big obstacle in their relationship. Their interactions would be filled with tension and awkwardness.
One common theme could be forbidden love. Since Zetsu has his own nature in the Naruto world and Sakura is a shinobi, their relationship might be seen as something that goes against the norms. Another theme could be the exploration of their different powers in relation to each other. For example, how Sakura's medical ninjutsu could interact with Zetsu's unique abilities.
One interesting sakura and zetsu fanfiction could be about them teaming up on a mission in a different dimension. They might face new and strange enemies, and through their combined efforts, they learn to understand each other better.
One good Sakura and Zetsu fanfic could be 'The Unlikely Alliance'. In this story, Sakura might find herself in a situation where she has to team up with Zetsu for a common goal. Maybe they are in a strange land and need to rely on each other's skills. Sakura's medical ninjutsu could come in handy when Zetsu gets injured, and Zetsu's knowledge of the area could help them navigate through difficulties.
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.'
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.
There could be many unknown authors writing such a fanfiction. It might be a new and emerging writer in the Naruto fanfiction community who just had this interesting and somewhat out - of - the - box idea. It's really hard to determine a specific author without more information.
One possible plot could be that Kushina has some misunderstanding about Sakura. Maybe she thinks Sakura is being disrespectful to Naruto in some way, so she punches her to teach her a lesson. Then, Sakura tries to figure out why Kushina is so angry and they start a series of interactions that eventually lead to them resolving their differences.
You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Use the search function and type in 'sakura and zetsu' to see what comes up. Check the ratings and reviews of the stories to find the good ones.