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.
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.
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 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.
A potential plotline is that Naruto and Yoruichi discover a hidden power within their relationship. This power could be something that was prophesied long ago in both their worlds. As they explore this power, they attract the attention of various factions who either want to use it for their own gain or are afraid of its potential. They also have to deal with the normal challenges of married life, like misunderstandings. For instance, Naruto might accidentally use a jutsu that Yoruichi doesn't understand, leading to a small fight that they have to resolve.
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.
A potential plotline could be Naruto arriving in the Highschool DxD world and being mistaken for a new type of devil or other supernatural being due to his 'dark godlike' nature. He would then have to prove his intentions and find his place in this new world. Another plotline could involve Naruto using his skills to train the characters in Highschool DxD in combat, similar to how he trained his friends in the Naruto world.
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.
One plotline could be that Naruto, in his typical bumbling way, tries to impress Kurenai with his new jutsu or skills, thinking it'll make her like him. He might accidentally make a fool of himself in the process but still manage to charm her with his determination. Another could be that Naruto misunderstands some advice about showing respect to elders and ends up flirting with Kurenai thinking it's the right thing to do.