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.
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.
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.
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 negative impact is that it can make fans who love the original characters feel disgusted. These characters are often created with certain personalities and storylines in the original work, and sexualizing them in this way is disrespectful.
A plot where Danzo fakes his death and teams up with Naruto undercover to take down a hidden threat to the village. They have to keep their alliance a secret from everyone, including the other village leaders. This creates a lot of tension and secrecy in the story.
One popular 'danzo crossover fanfiction' might be those that cross him with characters from other ninja - themed series. For example, a crossover with Naruto and another popular anime's ninja - like characters could create interesting scenarios. It could explore how Danzo's strict and secretive nature would interact with different ninja cultures and abilities.
Well, in such a fanfic, Naruto might turn against his friends. He could be made to use his power for Danzo's often shady purposes. For example, he might be forced to target those Danzo sees as threats to Konoha in a more brutal way than he normally would. His relationships would be severely damaged as his friends would be confused and likely try to stop him, not realizing he's been brainwashed at first.