She could try to appeal to Sasuke's emotions. If she can remind him of their past, of the times they fought together and cared for each other, it might make him release her. Sakura could talk about their shared experiences and how they are on the same side deep down, hoping to break through whatever is making Sasuke hold her captive.
Some fanfictions might play with the idea of alternate universes. In an alternate universe, the rules of ninjutsu are different. Naruto could be born with a combination of Uzumaki and Uchiha bloodlines, which would make it possible for him to use Susanoo. Or perhaps he is given this power by a powerful entity in the fanfiction for the purpose of fighting a greater evil that no one else can defeat.
One possible fanfiction could be about Sasuke using his Susanoo to save Sakura from a dangerous enemy. In this scenario, Sasuke, seeing Sakura in peril, quickly activates his Susanoo. The powerful, spectral form of the Susanoo reaches out with its massive hand and grabs Sakura, pulling her to safety. This moment could be a turning point in their relationship, with Sakura realizing anew how much Sasuke still cares for her, despite their past differences.
Yes, it is. Fanfiction often bends the rules of the original story. Writers can come up with all kinds of creative explanations to give Naruto the ability to use Susanoo.
One possible scenario in this fanfiction could be that Sasuke's Susanoo hand captures Sakura in a moment of intense battle. Maybe it's not an act of aggression but rather an attempt to protect her from an unseen enemy. The Susanoo's power is so great that when it reaches out to grab Sakura, it creates a sense of both danger and safety at the same time. She might initially be startled but then realizes Sasuke's intention.
There could be a fanfiction where Sasuke grabs Sakura with Susanoo during a chaotic event like a village - wide attack. The Susanoo's large form is able to cut through the chaos and reach Sakura just in time. Afterward, they have to work together to fend off the attackers. This shared experience could strengthen their relationship and make them more open to each other about their feelings and intentions.
In some fanfictions, Naruto might use Susanoo through some sort of power - sharing or fusion. For example, if there is a plot device that allows him to access the power of the Uchiha, he could potentially use Susanoo. Maybe a powerful enemy forces him to find new ways to power up and he somehow taps into this ability.
In this fanfiction, it might be that Sakura is physically restrained within Sasuke's Susanoo hand. She could be struggling against the power of the Susanoo itself, like it's draining her energy or causing her pain. And she has to find a way to either escape or get Sasuke to release her on his own.
One possible reason could be that Sasuke is being influenced by some dark force. Maybe it has corrupted his mind and made him see Sakura as a threat to his new goals, whatever those might be in this fanfiction's plot. So, he uses Susanoo to hold her captive as a way to neutralize that threat.
Sakura could struggle against the Susanoo hand at first, thinking Sasuke is trying to harm her. But once she realizes his intention, she could use her chakra to enhance the Susanoo's power. They could then combine their forces and launch a counter - attack against their opponents.