Well, it could be that Danzo trains Naruto in some ancient and forbidden jutsu. Naruto, being the curious and determined character he is, is intrigued by these new powers. However, he soon discovers that these jutsu come with a price. He has to struggle with the moral implications of using such powerful and potentially dangerous techniques while also trying to keep his identity as a jinchuriki a secret. This would create a lot of internal conflict for Naruto and also change his relationship with Danzo throughout the fanfic.
Naruto's character would likely become more complex. Danzo's training methods are strict and often involve a certain ruthlessness. Naruto, used to more open - minded and friendly training from the likes of Jiraiya, would have to adapt. He might become more calculating and less impulsive, which could be both good and bad. Good in that he'd be a more strategic fighter, but bad in that he might lose some of his charm and spontaneity that his friends love him for.
There could be a fanfiction where Danzo trains Naruto after realizing that Naruto's unique chakra and ability to make friends could be harnessed for the Root's purposes. However, Naruto being Naruto, he would try to influence Danzo to use these skills in a more positive and moral way, leading to an interesting dynamic between the two characters.
One possible plot could be that Danzo, despite his controversial nature in the original Naruto series, sees potential in Naruto. He might train Naruto in the ways of stealth and intelligence gathering. This could lead to Naruto having a more diverse skill set, different from what he learned from Jiraiya. Maybe Danzo would teach Naruto some of the Root's unique jutsu, which Naruto could then adapt to his own style, making him an even more formidable ninja.
You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). Just type in 'Danzo trains Naruto' in the search bar, and it should show you relevant stories.
The training could make Naruto more self - reliant. Danzo would probably push Naruto to rely on his own skills rather than always depending on his friends or the power of the Nine - Tails. This could lead to Naruto developing new techniques and strategies on his own. He'd grow as a ninja in a way that's different from his growth under other mentors, like Kakashi or Jiraiya. His confidence would also come from his own achievements in training rather than just his natural abilities.
It could also be due to Naruto's outcast status in the village initially. Danzo might think that banishing Naruto would be an easy decision as Naruto doesn't have many supporters at first. He might believe that by getting rid of Naruto, he can maintain the status quo in the village and prevent any disruptions that Naruto's presence might cause in the future.
There's a fanfic where Danzo tries to manipulate Naruto for his own political gain. However, Naruto, being the perceptive character he is, quickly catches on. This leads to a battle of wits between them. In the end, Naruto uses his own brand of talk - no - jutsu to make Danzo see the error of his ways and they end up working together to reform the village's political system.
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.
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.