In some fanfictions, the Akatsuki members might become allies of Dark Naruto. They are all outcasts in a way and have their own vendettas against the ninja world. Dark Naruto's goal of revenge on the Hidden Leaf could align with their overall goals of causing chaos and disrupting the existing power structure. For example, Itachi might see a different side of Naruto in this dark form and decide to support him for the sake of his brother Sasuke who is allied with Dark Naruto.
Sasuke is often an ally. Since Sasuke also has his own issues with the village and a thirst for power, he may see Dark Naruto as a kindred spirit. They could team up to take on the Hidden Leaf together. Another common ally could be Orochimaru. Orochimaru is always interested in power and the unique abilities that Dark Naruto might possess. He could offer his knowledge and resources to help Dark Naruto in his revenge plot.
One of the most common allies in Naruto gains power fanfiction is Hinata. She has always been in love with Naruto, and when Naruto starts to gain power, she is right there by his side. She might use her Byakugan to help Naruto in battles or in his power - exploration. Another ally could be Shikamaru. His intelligence is a great asset, and he could help Naruto strategize how to use his new powers most effectively. Additionally, the Sage of Six Paths is often involved as an ally. He might appear to Naruto in visions or directly in person to guide him in his power - gaining journey, teaching him about the deeper secrets of chakra and the ninja world.
I think Hinata could be an ally. She has always been in love with Naruto and would not stand by while he is in danger from Dark Naruto. Additionally, Shikamaru is smart and strategic. He might see the threat of Dark Naruto to the village and decide to assist Naruto, using his intelligence to come up with battle plans.
Sasuke could be an ally. He also has his own issues with Konoha and would understand Naruto's need for revenge. They could combine their powers, with Naruto's massive chakra reserves and Sasuke's unique Sharingan abilities.
One potential ally could be Neji. Neji has his Byakugan and his skills in combat. He could see the dark swordsman as an equal in terms of combat ability and form an alliance. Also, Tenten, who is fascinated with weapons. The dark swordsman's sword and his skills with it could draw Tenten's interest, and she might become an ally. They could exchange knowledge about different weapons and fighting techniques.
Sakura and Sasuke are very likely allies. Sakura has her medical skills and combat abilities, and Sasuke, despite his past issues, has great power. They've been comrades with Naruto for a long time.
One common plot is that Dark Naruto has been wronged by the Hidden Leaf Village in some way. Maybe he was falsely accused of a crime or treated unfairly due to his power. So, he decides to seek revenge on those who wronged him. He might start by gathering power secretly, perhaps by training with some forbidden techniques or making alliances with unlikely characters. Another plot could be that he discovers some dark secrets about the village leaders and uses that knowledge as a weapon against them in his revenge plan.
In some fanfictions, Orochimaru might be an ally. He has always been interested in Naruto's power and could see an opportunity in Dark Naruto. Another could be Sasuke. If Sasuke is still on his revenge path and sees Dark Naruto as a way to bring down the village, he might team up. And there are also some fan theories that some of the outcast ninja from Konoha who have grudges might side with Dark Naruto.
Sakura would definitely be one of his allies. She's always been by Naruto's side, and in this fanfiction, she could help him with his disguises, like using her medical ninjutsu to create more realistic physical changes. Kakashi could also be an ally. His intelligence and experience would be valuable in planning Naruto's undercover missions. He could teach Naruto how to act more convincingly in different disguises.
I think Gaara could be an ally. Gaara has control over sand and is very powerful. He and Naruto have a mutual respect for each other. Additionally, Jiraiya. Jiraiya was Naruto's mentor and has a vast knowledge of different jutsu and combat techniques that could be used against Omega Shenron.
Sasuke would be a likely ally. They have a long - standing relationship and have fought side by side before. Sasuke's Sharingan and his own unique abilities would be a great asset against the Ten - Tails.