Well, he might visit the dimension of the Sharingan's origin, where he could learn more about the Uchiha clan's secrets. Another could be a dimension full of powerful beasts, much stronger than the tailed beasts in his own world. And there could also be a dimension of pure chakra, where he can train and enhance his chakra control to an extreme level.
Well, it could be an ancient being that has been sealed away in a dimension for centuries. Naruto's arrival could break the seal accidentally, and this being, full of anger and a thirst for power, would become the main antagonist. Another option is a self - proclaimed god in a dimension who believes that Naruto's presence is an insult to his divinity. He would then try to eliminate Naruto. There could also be a group of anti - ninja fanatics in a dimension who view Naruto as a symbol of everything they hate about ninjas and would stop at nothing to get rid of him.
One potential antagonist could be a rogue ninja from another dimension who wants to use Naruto and Naruko's abilities for his own gain. He might be jealous of their power and try to trap them in different dimensions to study their techniques. Another could be a powerful entity that controls the portals between dimensions and doesn't want Naruto and Naruko messing around with the balance of different worlds.
One possible plot could be that Naruto and Naruko accidentally stumble upon a powerful jutsu or a strange artifact that opens a portal to different dimensions. In each dimension, they face unique challenges. For example, in a dimension where ninjas have different powers, they might have to learn new fighting styles to survive. They could also meet alternate versions of their friends and enemies, which would lead to interesting interactions and character development.
Maybe he discovers a special jutsu scroll left by an ancient sage. This scroll contains the secrets of dimensional travel that no one has been able to decipher until Naruto, with his unique determination and connection to the chakra of the world, figures it out.
One idea could be Naruto traveling to a dimension where ninjutsu doesn't exist, and he has to rely solely on his physical strength and strategic thinking to survive. Another could be a dimension filled with powerful, ancient beasts that Naruto must befriend or defeat. Maybe he travels to a dimension where everyone has a unique kekkei genkai that he has to learn to co - exist with.
He often meets versions of himself from different dimensions. These other Narutos might have different personalities or abilities due to the differences in their respective dimensions.
One possibility is that Naruto and Naruko are on a mission to save the multiverse in this fanfiction. They travel through dimensions to collect some special artifacts or powers that are scattered across different realities. Along the way, they might face challenges from the rulers of those dimensions who don't want them to take those things. This could involve epic battles and the forging of new alliances as they strive to complete their mission.
It could be some powerful dimension - hopping villains who want to use the power of different dimensions for their own evil purposes. They might see Naruto and Naruko as threats to their plans.
Perhaps Naruto travels to a dimension where everyone has a unique animal spirit companion. He has to bond with one of these spirits to gain access to certain powers. This would be a whole new experience for him as he's used to his own ninja world and its rules.