It could be a rogue ninja who has also discovered a dark version of the ultra instinct. This ninja uses his power for evil, like taking over villages and enslaving people. He would be a major threat to Naruto and his friends because his power is so similar yet so different from Naruto's ultra instinct - like powers.
A powerful and corrupt shinobi from a long - lost clan could be the main antagonist. This shinobi has been researching and experimenting with forbidden jutsus for years and finally unlocks a power that is a corrupted form of ultra instinct. He wants to use this power to rule the ninja world and eliminate all those who oppose him. He would see Naruto's ultra instinct as a threat and would constantly be trying to undermine and defeat him in various ways, such as by turning Naruto's friends against him or setting up elaborate traps.
Voldemort could still be the main antagonist. Even with Harry having the ultra instinct ability, Voldemort's dark magic and his army of Death Eaters would pose a significant threat. His cunning and thirst for power would make him a formidable foe against Harry's new power.
Another possibility is a rogue Saiyan from an alternate timeline. This Saiyan could be jealous of Goku's status as the strongest Saiyan in his own timeline. So, they use time travel to come to Goku's timeline and try to defeat him and take over as the dominant Saiyan.
It could be a long - lost Uchiha clan member who has a killer instinct amplified by a dark jutsu. This character has a vendetta against the Hidden Leaf for past wrongs done to the Uchiha. They use their Sharingan in very cruel ways, not hesitating to kill even innocent bystanders to achieve their goals.
One thing that could happen is that Naruto would become much faster in combat. Ultra Instinct gives the user incredible speed and reflexes. So Naruto could dodge attacks that were previously impossible for him to avoid. For example, he might be able to effortlessly avoid Sasuke's Chidori in a new and intense battle between them.
There are indeed Naruto fanfictions where he has Ultra Instinct. Fanfiction writers like to experiment with different power combinations. By giving Naruto Ultra Instinct, they can create really exciting battle scenes. For example, Naruto could use Ultra Instinct to dodge attacks more effectively during a fight with a powerful enemy like Madara, adding a whole new dimension to the story.
Another challenge is adapting his fighting style. Naruto's existing fighting style is based on his ninja training and the use of chakra. Ultra Instinct is a completely different concept. He has to figure out how to integrate the new power into his attacks and movements without losing his own identity as a ninja. For instance, he can't just abandon his signature Rasengan, but needs to find a way to use it in conjunction with Ultra Instinct abilities.
Maybe it could be a new dark wizard or witch who has a unique power that confounds Harry's rational thinking. This antagonist could be someone who uses mind - games and illusions, making it difficult for Harry to use his normal knowledge to fight back. So, he has to rely on his instincts more than ever to figure out their true intentions and defeat them.
Often, it begins when Naruto has a moment of deep self - reflection. He realizes the limitations of his current power and starts to explore new ways to fight. In the context of the fanfiction, this exploration could lead him to the path of learning Ultra Instinct. For example, he might encounter a wise character who gives him some cryptic advice that sets him on this journey.
Yes, it can be a great concept. Combining Naruto's ninja skills with the Ultra Instinct concept from Dragon Ball can lead to some really exciting and unique storylines. Naruto already has his own chakra - based powers, and adding Ultra Instinct - like abilities would take his power level to a whole new dimension. It could also create interesting interactions with his friends and foes as they react to this new power.
In this Ultra Instinct Batman fanfiction, there could be a moment where Batman has to teach others how to access a similar state. Maybe he trains Robin or some of the other young heroes in Gotham. This would not only show his growth as a character but also how he is able to pass on his knowledge and skills. It could also lead to the formation of a new and more powerful team in Gotham to fight against the ever - increasing threats.