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.
I think Kakashi could be an ally too. He has a wealth of combat experience and his Sharingan (even if it's not as powerful as Sasuke's in some ways) could come in handy. He could use his strategic mind to plan attacks against the Ten - Tails, and his various jutsu like the Lightning Blade would be useful in battle.
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.
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.
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.
Lucy might also be an ally. She has her celestial spirits that could be a great help. For example, some of the spirits like Leo could be very powerful in combat against the dragons. Also, Erza would be a great addition. With her various armors and combat skills, she could hold off a large number of dragons while Natsu focuses on the more powerful ones among the 10000.
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.
It's hard to say. Naruto has his powerful Nine - Tails Chakra and Sage Mode. But the five kages are also very strong. However, Naruto has a habit of coming out on top in tough situations, so he might be the likely winner.
It really depends on the context. If it's a physical fight based on brute strength, Naruto would likely have the upper hand. He has access to the Nine - Tails' chakra which gives him superhuman strength. But if it's a fight that involves strategy and medical ninjutsu, Sakura could potentially win. She's a very intelligent fighter and her medical skills could be used in a creative way to disable Naruto.
In addition to Naruto, other characters like Sasuke could also be main characters in this type of fanfiction. Sasuke has always had a complex relationship with Naruto and the power of the Ten - Tails could bring new challenges and opportunities for their relationship. Also, characters like Kakashi who are important in Naruto's life might also play significant roles.