One plotline could be that Naruto finds Zangetsu during a mission in a hidden part of the ninja world. He then has to keep it a secret from his village while he tries to figure out how to use it. Another could be that an enemy uses some sort of dark magic to force Zangetsu onto Naruto, and he has to learn to control it to break free from the enemy's control.
A potential plotline might involve Naruto being sent to the Bleach universe accidentally. There, he meets Zangetsu and forms a bond with it. When he returns to his own world, he has to adapt Zangetsu's power to the ninja way. Or perhaps Zangetsu appears in Naruto's world during a time of great crisis, and Naruto is the only one who can wield it to save everyone.
Yes, there are likely fanfictions out there with this concept. The Naruto fandom is vast and creative, and crossovers or unique concepts like Naruto having White Zangetsu are not unheard of. You can start by searching on popular fanfiction websites.
One potential plotline could be Naruto's discovery of the Shikotsu Myaku. He might initially be confused about this new power within him and have to figure out what it is and how to control it. Another could be that enemies become aware of his new power and target him specifically because of it.
Perhaps in this fanfiction, Naruto is given White Zangetsu as a gift from a mysterious figure. With it, he discovers new powers and abilities. He might have to train hard to master it, and in the process, he could face challenges not only from his own world but also from forces that are attracted to the power of White Zangetsu. This could lead to new alliances and enemies for Naruto, completely changing the dynamics of the Naruto universe.
One potential plotline could be Naruto using his new powers to finally bring true peace to the ninja world. He could use the Sharingan to understand the root causes of conflicts by seeing through people's true intentions and the Rinnegan to reshape the world in a more harmonious way.
One plotline could be Naruto being transported to the Mandalorian universe. He has to learn the ways of the Mandalorians while still relying on his ninja skills. Another could be a Mandalorian coming to the Naruto world and Naruto teaming up with him, sharing their combat knowledge. For example, Naruto could teach the Mandalorian about chakra, and the Mandalorian could teach Naruto about their advanced weaponry.
Well, a possible plotline is that Naruto has to deal with the fact that he has the power of the Kyuubi in a more innate way. Maybe he discovers that he can access certain abilities that are unique to being the son of Kyuubi, like a deeper connection to the chakra of the nine - tailed beast. There could also be a plotline where the Akatsuki target him not just for the Kyuubi's power but because of his special connection to it as its son. And his own village might have to decide whether to protect him or sacrifice him for the greater good.
One plotline could be that Naruto is tricked into believing the Jedi are hiding a great power that could save his world but are being selfish about it. So, he decides to steal it from them, which is seen as a betrayal. Another could be that he falls in love with a character who is an enemy of the Jedi, and for love, he turns against them.
Maybe a group of villains in the RWBY fanfiction are trying to summon Dark Naruto to use his power for their own evil purposes. The RWBY heroes find out about this plan and have to stop the summoning. Along the way, they meet some characters from the Naruto universe who are also trying to prevent the misuse of Dark Naruto's power.
One potential plotline could be Naruto being summoned to the world of Rias by accident. He then has to figure out how to get back home while also getting involved in the power struggles within the devil world. Maybe he becomes a key player in a battle for control of a certain territory.
One plotline could be that Black Rock Shooter is summoned to the Naruto universe by accident. She then has to adapt to this new world with its own set of rules and powers. Another could be that Naruto travels to a world similar to Black Rock Shooter's and has to team up with her to survive. Maybe there's a powerful enemy in both their worlds that they need to defeat together.