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.
One possible plot could be that the Juubi somehow gets transported to the Naruto world during a strange rift in space - time. Naruto and his friends have to figure out how to deal with this powerful and chaotic entity. They might initially try to fight it, but then realize they need to find a way to seal it again, like they did with the other tailed beasts. This could lead to new alliances and power - ups for the characters as they struggle to protect their world.
Well, first off, you need to pick a central conflict that involves all three elements. It could be something like a dark magic user in Fairy Tail is trying to harness the power of the Juubi in the Naruto world. Then, develop your characters. Let's say Gray from Fairy Tail discovers he has some latent chakra abilities due to his exposure to the Juubi. For Naruto characters, they could be intrigued by the different forms of magic in Fairy Tail. Also, don't forget about the settings. You can describe the Hidden Leaf Village as it appears to the Fairy Tail mages and vice versa. This kind of cross - over exploration can make your fanfic really stand out.
In this kind of fanfiction, Naruto becoming the Juubi would completely change the Naruto universe. It would mean he has an insane amount of power. He could potentially reshape the world, bring peace in a way no one expected. His relationships with other characters like Sasuke would also be very different. Sasuke might be both in awe and a bit jealous of this new power Naruto has.
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.
A common theme is the clash of different power systems. Naruto has his chakra and various jutsu, while Inuyasha has his demon powers. When the Juubi is added into the mix, it creates a complex power dynamic.
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.