In a more creative fanfic, an ancient ninja who was the originator of the Chidori technique could appear. This ninja could be brought back to life through some sort of magic or jutsu. He sees Naruto's pure heart and decides to pass on the Chidori knowledge to him, believing Naruto will use it for good.
Maybe Sasuke teaches him the Chidori in a moment of camaraderie. And for the Rasengan, it could be an older and wiser Naruto from the future who travels back in time to teach his younger self.
The signs are quite obvious in some fanfictions. Sasuke will often interfere when Sakura is making plans with others. He'll try to change the plans so that it involves just the two of them. And he has a habit of giving death glares to anyone who he thinks is getting too friendly with Sakura. This shows his strong possessive nature towards her.
If she starts showing more interest in other types of fanfiction or non - fanfiction works. For example, she might be more into fanfiction about other Naruto characters or completely different stories from other series. This shift in focus indicates her loss of interest in Sasuke fanfiction.
One common sign could be excessive control. For example, if Sasuke in the fanfiction constantly restricts Sakura's actions or who she can interact with. Another sign might be verbal aggression, like Sasuke yelling at Sakura in a demeaning way. Also, if there's a pattern of Sasuke ignoring Sakura's boundaries, such as physically getting too close when she doesn't want to, it can be a sign of an abusive relationship in the fanfiction.
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.
In a fanfic where Naruto knows the Chidori, it could change the power dynamics in battles significantly. Naruto's already vast chakra reserves combined with the piercing and fast - acting nature of the Chidori would make him an even more formidable opponent. He could use it in close - quarter combat situations, perhaps integrating it with his Shadow Clone Jutsu in creative ways. For example, he could have clones perform the Chidori from different angles, catching his enemies off - guard.
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.
In fanfiction, Naruto creating Chidori could be possible through some creative plot devices. Maybe he was taught by Sasuke in an alternate universe where they teamed up earlier. Or perhaps he discovered a scroll that had the knowledge of Chidori and was able to learn it on his own with his determination and quick - learning ability.