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.
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 could also be an ally. As a medic - ninja, she could help Kagome if she gets injured. And Sakura is used to dealing with strange situations and new people, so she would be open to helping Kagome.
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 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.
In a 'naruto omega shenron fanfiction', it could be interesting if Omega Shenron has the power to corrupt chakra. Naruto then has to not only fight against Omega Shenron physically but also resist the corruption of his own chakra. He might have to go on a spiritual journey to find a way to purify his chakra and defeat the dragon.
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.
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.
Sakura could be one of his allies. She's intelligent and quick to learn, so she might pick up on the basics of alchemy from Naruto and help him in his alchemical endeavors. She could use her medical knowledge along with alchemy to create new healing techniques.
Iruka sensei might also be involved. He has always been like a father figure to Naruto. He could give Naruto the confidence he needs and maybe even help him plan the location for the proposal, perhaps a place that has sentimental value for both Naruto and Hinata related to Naruto's early days at the academy.