Maybe a new character, created specifically for the fanfic. This character could be a rogue ninja who has heard rumors about the power of the Forbidden Scroll and wants to use it to take over the ninja world.
Maybe he's tired of being underestimated. Naruto has always been looked down upon in the village. Stealing the Forbidden Scroll could be his way of taking matters into his own hands and showing everyone that he's capable of great things. He might think that the secrets in the scroll will give him the edge he needs to change people's opinions of him.
Well, it could be about Naruto finding the Forbidden Scroll and uncovering some long - lost jutsu that was sealed away for a reason. Maybe it's a jutsu that's too powerful or has some dark side - effect.
Jiraiya could be a crucial ally. As Naruto's mentor, he would be interested in the knowledge of the Forbidden Scroll. He could guide Naruto on how to use the knowledge safely and also protect Naruto from those who would try to take the scroll from him.
Well, in the fanfic, Naruto might use the knowledge from the Forbidden Scroll to develop some unique and powerful jutsu. He could gain an edge in battles against his enemies like Orochimaru or Pain.
If Naruto steals the Forbidden Scroll in the fanfic, it's likely to set off a chain of events that will test his loyalty to the village. On one hand, he may be tempted to use the powerful techniques in the scroll for his own gain, like to get stronger quickly to defeat his enemies. But on the other hand, his sense of right and wrong, which has been instilled in him by Iruka and others, might make him realize that he has to return the scroll. Or, he could try to find a middle ground, where he uses just enough of the knowledge in the scroll to help the village in a time of need, without completely going against the village's rules.
Naruto is often a main character. Since he has a knack for getting into trouble and his determination to become stronger, he's likely to be involved with the Forbidden Scroll in some way. He might be the one who accidentally discovers it or has to protect it.
I'm not sure as I haven't read this particular fanfic. But it could have Naruto himself as a main character, since it's a Naruto - related fanfic. He might be on a quest to find or understand the scroll.
It could also lead to him being targeted by enemies. If word gets out that he has access to the Forbidden Scroll's secrets, villains might come after him to either steal the knowledge from him or stop him from using it. This would put him in a lot of dangerous situations and force him to grow up quickly in order to protect himself and the village.
Jiraiya might also be a key character. As Naruto's mentor, he could be the one to guide Naruto on whether or not to explore the secrets of the alternate forbidden scroll. He has a lot of knowledge about the ninja world and its taboos, so his presence would be important in this kind of fanfiction.
Another storyline could involve the Forbidden Scroll being stolen by an enemy of Konoha. This would set off a chain of events where Naruto and his friends have to embark on a dangerous mission to retrieve it. Along the way, they might uncover secrets about the scroll's origin and its true power, which could be far greater than they initially thought.