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.
Well, it really depends on how the fanfic is written. If the fanfic focuses on Naruto's weaknesses and Danzo's preparedness, Danzo could potentially win. He has a lot of political power and experience. However, if it follows the general power scale of the Naruto universe, Naruto has a better chance. His positive energy and the power of friendship (a bit cheesy but true in Naruto's world) often help him overcome his opponents. Naruto also has the ability to quickly adapt to new situations, which would be a big plus against Danzo.
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.
If we consider a Naruto - Danzo fight in a fanfiction, Naruto has several factors in his favor. His training with Jiraiya and other experiences have made him a formidable ninja. He can go into Sage Mode, which enhances his physical abilities and gives him access to more powerful jutsu. Danzo, while being a shrewd and experienced ninja, has his weaknesses. His over - reliance on the Sharingan and his somewhat underhanded methods might not be enough against Naruto's raw power and unwavering spirit.
Most likely Danzo would be seen as the main antagonist. His actions throughout the Naruto series were often self - serving and harmful to others, like his role in the Uchiha massacre. So when Naruto talks to him, he would be the one Naruto has to oppose or at least try to change.
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 be the other Hidden Villages. Since Naruto is becoming so powerful under Danzo's training, they might see him as a threat and try to stop him. Maybe they would send their best ninjas to take him down.
Naruto himself would be an interesting addition. His ideals of friendship and forgiveness could be in direct contrast to Danzo's more underhanded and secretive ways. It would create a lot of tension and potential for character growth for both of them.