In a fanfic where Danzo gives Naruto the Sharingan, it could lead to a very different Naruto. Naruto might gain new abilities and a new perspective on the Uchiha. He could potentially master the Sharingan quickly with his strong willpower, but he would also have to deal with the dark side of the Sharingan, like the curse of hatred that often comes with it.
A fanfic plot where Danzo gives Naruto Sharingan could start with a life - or - death situation. Danzo sees Naruto as the only hope and gives him the Sharingan to give him an edge. Naruto then has to quickly adapt to the new power. As he does, he starts to have visions related to the Uchiha's past through the Sharingan. These visions lead him on a quest to uncover the truth about the Uchiha massacre and Danzo's role in it. This could lead to a conflict within the village as Naruto uncovers dark secrets and has to decide what to do with the knowledge and his new - found power.
Naruto would become much more powerful. The Sharingan could give him an edge in battles, allowing him to copy jutsu more easily. He might be able to use it to analyze his opponents faster. However, he would also have to deal with the negative aspects of the Sharingan, such as the potential strain on his eyes and the risk of overusing it.
There would be several implications. Firstly, Naruto's training would take a new turn. He would need to learn how to control and use the Sharingan properly, which could be a difficult task considering his already existing training regime. Secondly, politically, it could cause a stir in the Hidden Leaf. The Sharingan is a symbol of the Uchiha clan, and Naruto getting it from Danzo (who had a controversial relationship with the Uchiha) could lead to various political debates. Thirdly, on a personal level, Naruto might experience new emotions and temptations that come with the Sharingan's power. He might have to struggle with the darker aspects of the Sharingan, such as the potential to be consumed by its power, just as some Uchiha did in the past.
It's possible in the realm of fanfiction. Danzo is a complex character with access to many secrets and resources. If he were to give Naruto a Sharingan, it would completely change Naruto's power dynamics. Naruto already has his own powerful abilities like the Nine - Tails' chakra, and adding the Sharingan would make him an even more formidable opponent. However, this would also bring new challenges for Naruto in terms of controlling the Sharingan's powers and dealing with the Uchiha - related baggage that comes with it.
In the Naruto universe, while it's highly unlikely in the official canon, in fanfiction, anything can be made possible. The Sharingan is a kekkei genkai exclusive to the Uchiha clan. Danzo had Sharingan in his arm which he obtained through rather unethical means. For Naruto to get it from him in fanfiction, it could be written through some sort of experimental jutsu gone wrong or right. Maybe a jutsu that was meant to transfer power ended up giving Naruto the Sharingan. However, this would completely change Naruto's power dynamics and character arc as the Sharingan comes with its own set of abilities like the ability to copy jutsu, predict movements, and use genjutsu.
He'll likely lose his power and influence in the village. Once Naruto reveals his secrets, the villagers won't trust him anymore. His actions might be put on trial, and he could be exiled from the village.
One possible motive could be to gain control over Naruto's power. Since Naruto has the Nine - Tails within him, Danzo might want to use that power for his own political or military gains. He's always been power - hungry and sees Naruto as a tool to achieve his goals of making Konoha stronger in his own way, which might involve more underhanded and controlling methods.
It could also be a completely new character introduced into the fanfiction. A foreign ninja who was inspired by some of Danzo's ideas (not the bad ones) and comes to Konoha to take on the role of successor, bringing in new perspectives and ways of doing things.
One possible motive could be revenge. Maybe the oc or someone close to the oc has been wronged by Danzo in the past. So, to seek justice, the oc decides to take him down.