Naruto could be one of his allies. Naruto has this knack for seeing the good in people and would probably be willing to give Obito a chance to redeem himself. Also, Sakura, with her skills as a medic - ninja, could be an ally. She might help heal Obito if he was injured during the war and see the potential in him to be a force for good.
In such a fanfic, perhaps Obito could use his Sharingan powers in new and unexpected ways after the war. He might start to see the world differently and strive to make amends for his past actions. He could use his abilities to help rebuild the villages he once tried to destroy, like Konoha.
There could also be interactions with the new generation of ninjas. They might view Obito as a sort of legend or a cautionary tale. Their fresh perspectives could either inspire Obito to change further or make him feel even more out of place in this new post - war ninja world.
Besides Obito, Sasuke could be a main character in this fanfiction. Sasuke, with his own journey of redemption and his understanding of power and the ninja world, could have an interesting dynamic with the reincarnated Obito. Also, Sakura might play a role. She could be the one who tries to heal the reincarnated Obito, both physically if he is injured during his new adventures and emotionally from the traumas of his past life.
The main antagonist could be Madara. Even though Obito was once following Madara's plan, with Naruto joining Obito, Madara might see this as a threat to his grand scheme. He could try to manipulate events to drive a wedge between Naruto and Obito or directly attack them to stop their new alliance from achieving their goals.
Well, in this kind of fanfiction, Obito might face a lot of challenges in readjusting to the post - war world. Maybe he has to deal with the guilt of his past actions.
In such a fanfiction, Obito might seek redemption. He could use his remaining powers to help rebuild the villages he once damaged. Maybe he would start by teaching the new generation about the true cost of war and how to avoid the same mistakes.
Well, in this fanfic, Obito might raise Naruto to be more cynical about the Hidden Leaf Village. Since Obito has his own grudges against the village, he could influence Naruto's perception of it. However, Naruto's own kind heart might still shine through and he could end up trying to change Obito's views while still learning from him. For example, Naruto could use the power and skills he learns from Obito to protect the village in a new way, bridging the gap between Obito's distrust and the village's need for protection.
In a 'naruto joins obito fanfic', it could start with Naruto being transported to a parallel dimension where Obito has already started his own ninja village. Naruto, seeing the potential in Obito's ideas but also the flaws, decides to join him. He helps Obito to correct his path. For example, Naruto could teach the ninjas in Obito's village about the importance of hope and not giving in to hatred. They could then work together to build a village that is a model for the rest of the ninja world, based on both Obito's and Naruto's ideals.
It could be that Naruto jumps in front of Obito in a moment of desperation. He might not have any other option to stop Obito's immediate attack. This action could be a turning point in the story. It could inspire other characters to join Naruto in his fight against Obito. Also, it could show Obito the strength of Naruto's will, which might make him question his own motives and actions.
Well, the reborn character could explore Obito's Sharingan powers in new ways. Instead of using them mainly for combat like Obito did in the original story, they could focus on using the Sharingan to understand the emotions of others. For example, they could use it to detect lies and hidden motives, which would be a unique twist on the power.