Well, in some versions, Naruto might use his influence among the villagers. After Pain's attack, the village is in a state of rebuilding and re - evaluating. Naruto, being a hero, has the ears of the people. He could start by presenting evidence of Danzo's underhanded deals, like his connection to Orochimaru perhaps. Maybe he found some secret documents during the chaos that Pain caused, which link Danzo to unethical experiments or power - grabbing within the village.
It depends on the specific fanfic. Some authors might come up with this plot twist to explore different character motivations and conflicts. In others, it might not be the case at all.
In this fanfic, the oc could bring Danzo down by winning the loyalty of those who were previously under Danzo's command. If the oc is able to show these people that Danzo's ways are wrong and offers a better alternative, they might turn against him. This would weaken Danzo's power base and eventually lead to his downfall. Additionally, the oc might be able to use Danzo's own paranoia against him. By creating situations that make Danzo act rashly, the oc can expose his true nature to the rest of the village.
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.
Well, it could be through having some unique jutsu or abilities. Maybe the oc has a special power that can counter Danzo's Sharingan-based techniques or his political influence within the Hidden Leaf. For example, if the oc has a power to expose all of Danzo's secrets at once, it would undermine his authority and bring him down.
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.