Sakura could be a great protagonist. She's already a strong character in the Naruto universe, and if she were to somehow be involved with the Juubi eye in a crossover, it would be interesting to see how she'd handle the new power. Maybe she could use it to enhance her medical ninjutsu in ways never seen before.
I think Naruto himself could still be a good protagonist even in a crossover with the Juubi eye. He's always been about breaking boundaries and achieving the impossible. With the addition of the Juubi eye, he could face new challenges and enemies. He might have to struggle with the darker aspects of the Juubi eye's power, as it's a very powerful and potentially dangerous ability. But, as we know Naruto, he'd try to find a way to use it for good and protect his friends and the village.
A character from another anime or game could be a good protagonist in this fanfiction. For instance, if a character like Ichigo from 'Bleach' were to cross over into the Naruto world and gain the Juubi eye. He already has his own set of powers, and combining them with the Juubi eye would create a really unique power dynamic. He could bring his sense of justice from his own world and apply it to the situations in the Naruto world while dealing with the new power.
One possible interesting plot could be a crossover where a character from another universe gains the Juubi eye in the Naruto world. This new power could disrupt the balance of power, leading to alliances and rivalries being formed in new and unexpected ways.
It could be a powerful ninja from Naruto's world who has been corrupted by the Juubi's power and somehow ended up in the Inuyasha world. He wants to use the power of the demons in the Inuyasha world to make himself even stronger and take over both worlds.
Definitely Hinata. Since she has feelings for Naruto in the original series, she could be a part of the harem. Sakura might also be there as she is an important female character. From the Juubi - related side, maybe a female form of one of the tailed beasts. Also, Tenten could be included as she is a strong female ninja. And perhaps a mysterious female character who has a special connection to the Juubi's power.
Naruto would likely be one of the main characters. Since it's a Naruto - centered fanfiction, he'd be the one using the Juubi eye for time travel, probably to save the world or his friends. Sasuke could also be involved. Maybe he initially opposes Naruto's use of the Juubi eye for time travel but later realizes its importance. And there could be villains like Kaguya who might try to stop Naruto from using the Juubi eye's time - travel powers for their own evil purposes.
Madara Uchiha could be a great antagonist in this fanfic. Even though he was defeated in the original Naruto story, if he somehow gets resurrected and gets a chance to control the Juubi, he would be a formidable foe. He already had grand plans for the world in the original story, and with the Juubi's power, he could be even more dangerous. He might want to reshape the world according to his vision, and Naruto would have to fight him once again to protect everyone.
Naruto becoming the Juubi in a crossover fanfiction could lead to new alliances. Other characters from different series might see him as a valuable ally or a threat. For instance, characters from Bleach might approach Naruto differently. And Naruto himself might have to decide who he can trust with his new power.
Well, it could be about Naruto having to face a new and more powerful form of the Juubi. Maybe the Juubi has some new abilities or is being controlled by a new villain. Naruto would then have to gather his friends and allies, train harder, and discover new powers within himself to defeat this new threat.
One possible plot could be that the Juubi from Naruto somehow breaks through dimensions and ends up in the Bleach universe. This could cause chaos as the spiritual energy in the Bleach world reacts to the Juubi's massive chakra. Ichigo and Naruto might team up to stop the Juubi. Naruto could use his sage mode and Rasengan techniques, while Ichigo uses his Bankai. They would have to learn to combine their powers and fighting styles to defeat the Juubi and send it back to its own universe.
Well, one possibility could be that Naruto, with the power of the Juubi, enters the DxD world. He might use his ninja skills and the Juubi's immense power to face off against the devils and other supernatural beings in the DxD universe. Maybe he forms alliances with some of the characters there, like Issei and his friends, and together they fight a common enemy that threatens both the ninja world and the DxD world.
The plot could involve the Juubi's chakra influencing some of the demons in the Inuyasha world. Naruto arrives and has to figure out how to stop this chaos. He might teach Inuyasha some of his ninja skills, and Inuyasha in turn could show Naruto how to deal with the demons. They could also find that the Juubi has some ancient connection to a powerful artifact in the Inuyasha world, and they have to stop an evil force from using this connection for their own gain.