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.
Well, in some fanfictions, the Nine - Tailed Fox could be an unexpected ally. Since it's inside Naruto and has its own interests, it might help Naruto against Dark Naruto. Also, the other members of Naruto's old team like Sakura might also join in. She has grown stronger over time and would want to support Naruto in this fight against his dark self.
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.
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.
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 could be a likely ally. Despite their past differences, Sasuke might see the injustice in Naruto and Shikamaru's banishment and decide to help them. He has his own reasons to oppose the village's wrongdoings at times.
Well, first of all, Jiraiya might return in some form, like a spirit guide or something, and be an ally to Naruto. Tsunade would also be there, using her leadership and medical ninjutsu. And then there are the new generation characters like Boruto. He could learn from Naruto and help out, seeing as he also has his own unique abilities. The Allied Shinobi Forces from the previous big battles might also reunite to support Naruto in saving the world.
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.
In Naruto fanfiction, Hinata is often seen as a better match for Naruto. Hinata has been in love with Naruto for a long time. She has always supported him silently from the sidelines. Her gentle and shy nature can be a good complement to Naruto's more boisterous personality. Sakura, on the other hand, had feelings for Sasuke for most of the story, which makes her connection to Naruto seem less deep in comparison.
It really depends on how the author writes it. If the black Naruto is written as being more powerful with no moral constraints, he might seem like the likely winner at first. But Naruto has the power of friendship and his strong will, so he could also come out on top.
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.