In this fanfiction, Naruto might use his unique jutsus like the Shadow Clone Jutsu. He could arrive just in time to stop an enemy from killing Zabuza. Maybe Naruto realizes Zabuza's potential for redemption and decides to protect him.
Perhaps it's a group of mercenary ninjas. They've been hired by someone who has a grudge against Zabuza. They see Zabuza as an easy target when he's alone or vulnerable, but Naruto shows up to spoil their plan.
His confidence could grow significantly. By being able to save two powerful ninjas, Naruto would prove to himself that he is capable of great things. He may start to believe more in his own abilities and take on more challenging tasks without hesitation. For example, he might be more willing to take on stronger opponents in future battles.
Well, in the original, Haku and Zabuza die. But in this fanfiction, Sakura's actions change that fate. This is a huge deviation as it completely alters the expected outcomes for those characters.
Zabuza could be quite cold to Naruto at the beginning. He's not exactly the warm - and - fuzzy type. But when Naruto shows his skills and tells his story of being banished unjustly, Zabuza might empathize on some level. He might not show it openly, but he could start to protect Naruto in his own way, perhaps because he sees a bit of his younger self in Naruto who was also wronged by the ninja system.
One possible plot could be that Sakura, being a skilled medic in the Naruto universe, comes across Zabuza and Haku who are severely injured. She decides to use her medical jutsu to save them. Maybe she's on a solo mission and her sense of morality makes her help them, despite them being former enemies. This could then lead to an interesting alliance or at least a mutual respect between them.
One interesting aspect could be the character development of Sakura. In the original series, she might not have had as much interaction with Haku and Zabuza. In the fanfiction, she could show new skills and a more assertive personality while saving them. For example, she might use her medical ninjutsu in creative ways to heal their wounds.
He could learn Zabuza's signature water - style jutsu, like the Hidden Mist Technique. This would give Naruto an advantage in terms of creating diversions and hiding his movements during battles.
Zabuza could be portrayed as more of a mentor figure. Instead of just being a ruthless assassin, he could be shown guiding Naruto in some aspects of combat or strategy. Maybe he survived his encounter with Kakashi and had a change of heart, and now he imparts his knowledge to Naruto who is now a sensei himself.
On a more practical level, Naruto's skills might be misused. Instead of using his power to protect Konoha and the people he cares about, he's using it for Zabuza's self - centered motives. This could also make him a target for other ninja who oppose Zabuza. So not only is he in danger from the people he used to call friends, but also from new enemies. And if he ever tries to break free from Zabuza, he might face severe retaliation from Zabuza himself, which could be life - threatening.