One plot idea could be that Sasuke sees Naruto getting closer to Sakura during a mission. He becomes possessive and starts to interfere in their interactions, creating tension within the team. For example, he might try to take over tasks that Naruto and Sakura are doing together just to be near Sakura.
Perhaps in this fanfiction, they are the only ones who can stop an ancient evil that has been awakened. They are inseparable in the sense that they are constantly protecting each other. Naruto might use his Rasengan to create openings, Sasuke with his Sharingan to analyze the enemy's moves, and Sakura to heal any injuries they sustain immediately, making them an unbeatable trio.
One plot idea could be that Kakashi notices other ninjas showing too much interest in Sakura during a mission. So, he starts subtly making it clear that she's under his protection. For example, he always pairs up with her during dangerous tasks and gives warning looks to those who get too close.
A plot could involve a situation where Gaara and Sakura are in a relationship, and Gaara's possessiveness is a result of his past traumas. He's afraid of losing Sakura like he lost so much in his childhood. So he constantly hovers around her, which at first Sakura finds a bit suffocating. But as she understands his reasons, she tries to help him deal with his insecurities. And in return, Sakura's possessive side shows when other female ninjas show an interest in Gaara. She might use her medical ninja skills to subtly show her superiority in taking care of Gaara.
How about a story where an unknown enemy creates clones of them? Naruto, Sasuke, and Sakura have to not only fight these clones but also figure out who the enemy is and why they are being targeted. Along the way, they might discover new powers within themselves and also learn more about their relationships with each other.
In a fanfiction, there could be a story where a mysterious enemy targets Sasuke, Naruto and Sakura specifically because of their unique bond. The three of them then have to go into hiding and figure out who the enemy is and what they want. Along the way, they rely on each other's strengths and grow even closer as friends.
Perhaps a fanfiction where Sasuke loses his memory and Sakura has to help him regain it. She takes him back to all the places that are important to their past, like their old training grounds. As she tries to jog his memory, they end up reliving old memories and emotions, and this could lead to a rekindling of their relationship in a very emotional way.
One plot idea could be that they grow up in a hidden village where they have to face a new and powerful enemy together. As siblings, they would have a stronger bond than in the original story, which could lead to some really intense and emotional fight scenes where they protect each other at all costs.
A plot could involve time travel. Naruto and Hinata find themselves sent back in time, and they have to find a way to get back to their own time. Along the way, they meet a younger Sasuke and Sakura. This gives Naruto a chance to try and prevent some of the bad things that happened in Sasuke's past, and Hinata can bond with a younger Sakura, sharing her own experiences of growing up shy and how she overcame it.
One plot idea could be that in an alternate universe, Sasuke and Sakura have been in a relationship for a while. Naruto, being their close friend, witnesses their first real kiss during a moment of celebration after a big victory in the village. This kiss could symbolize the stability and love they've found in each other, and Naruto's reaction could be a mix of happiness for his friends and a tinge of his own loneliness.
One idea could be exploring how Sasuke and Sakura adjust to the impending parenthood while also dealing with their ninja duties. Maybe Sasuke is away on a mission and Sakura has to handle the pregnancy alone for a while, which makes her stronger and more independent.