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 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.
One possible plot twist could be Goku traveling back in time to train with his past self. This would not only enhance his own power but also change the course of his past battles. For example, he could teach his past self Ultra Instinct earlier, leading to different outcomes against foes like Frieza or Cell.
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.
Yes, it is possible. In fanfiction, writers have the creative freedom to reimagine the origin of characters. Goku being born with Ultra Instinct can add a whole new layer to his character development. It could mean that his power growth would be different from the original story, perhaps faster or with different challenges along the way.
You could write a fanfiction where Goku teaches others about Ultra Instinct. Maybe he tries to pass on this power to Gohan or Trunks. This would involve Goku having to understand the power more deeply himself in order to teach it, and it would also be a great way to develop the other characters.
A fanfiction could focus on Goku's relationships while in Ultra Instinct. For example, how does he interact with his friends and family? Does Vegeta view him differently? In this story, Goku might struggle to control the Ultra Instinct around his loved ones because it makes him seem so detached. This could lead to some interesting emotional moments between the characters.
A group of rogue Saiyans from a parallel universe might also be antagonists. They see Goku's Ultra Instinct as a threat to their own plans of universal domination. They have their own unique powers and battle tactics, making them a formidable foe for Goku and his friends.
Frieza would be a great antagonist. If Goku goes back in time to a point before Frieza was defeated, Frieza could be an even more menacing force as Goku might not have all the power - ups he had in the original timeline. He'd have to rely on his wits and basic training more.
Well, he could change a lot of things. Maybe he'd prevent some of the big battles or save some characters who died in the original timeline. For example, he might stop Frieza from destroying Planet Vegeta earlier. It would also be interesting to see how his Ultra Instinct powers interact with the past events and characters who haven't seen such power before.
Yes, he does. Goku achieves Ultra Instinct in the manga.