Sasuke could be one of the characters to help Naruto. Their combined powers have been shown to be very effective in the original series.
In this fanfiction, perhaps Naruto breaks Kaguya by outsmarting her. He could use his quick thinking and knowledge of her weaknesses. He might form alliances with other characters in ways that give him an edge over Kaguya, and then use his signature Rasengan in a new and unexpected way to break her power.
Another possibility is that it could be a new character created specifically for the fanfiction. A character who wears a mask for some secretive purpose, and when the mask breaks, it sets off a chain of events that are unique to the fan - created storyline.
His friends like Sakura, who is a medic - ninja, would surely be the first ones to try to help. She could use her medical knowledge and skills to diagnose Naruto's illness.
Most likely Naruto and Sasuke. In the original series, they were already very powerful when they faced Kaguya, and in fanfiction, they could be even more so. Their combined strength and determination usually make them the favorites to win.
Well, I think Hinata would also be there to support Naruto. She's always been by his side and her Byakugan might be useful in detecting any strange changes in Naruto's body or chakra after breaking the seal. Also, Tsunade could play a role. As the Hokage, she has resources and knowledge. She could provide Naruto with protection if there are enemies targeting him because of the broken seal and also offer advice on how to handle the situation.
His friends like Sasuke and Sakura might help. They have different skills that could be useful.
Sasuke would be a likely helper. Despite their past differences, they are comrades now and Sasuke has his own set of skills that could be useful in building a village, like his Sharingan - assisted precision in construction work.
Maybe she wants to disrupt the balance in the ninja world. Naruto is a key figure in maintaining peace and unity. By kidnapping him, she can create chaos and make it easier for her to regain control over the world she once ruled.
There could be many unknown authors out there writing fanfictions. It's hard to say specifically who the author of this one would be without more research. It might be some Naruto - loving fan who wanted to explore a more tragic and angsty storyline with these characters.
Perhaps there is no traditional antagonist. Instead, Naruto's own insecurities could be the 'antagonist'. He's so used to wearing the mask of the happy - go - lucky guy that his inner self is resistant to being exposed. So, in a way, his own fear of change and vulnerability is what Sakura has to fight against when she tries to break his mask.