Well, it depends on the author's imagination. Some fanfics might show ultra instinct as an overwhelming power, while others might have a more nuanced take.
Ultra Instinct is depicted really powerfully and visually stunning in the manga. It shows the intensity and unique abilities of the characters.
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.
Todoroki might be another likely character. He has a complex power set and has been through a lot of emotional and physical growth. His struggle to control his two quirks could be a parallel to the control needed for Ultra Instinct. If he can master the balance within himself, it could potentially lead to the awakening of Ultra Instinct in him.
I'd recommend 'Ultra Instinct Reborn'. It not only showcases intense battles with ultra instinct but also has a captivating story that ties in nicely with the original dragon ball lore.
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.
In the manga, Ultra Instinct is not achieved by Vegeta. The power-up is mainly associated with Goku's development and storyline.
Yes, he does. Goku achieves Ultra Instinct in the manga.
Yes, Goku has mastered Ultra Instinct in the manga.
Typically, in izuku ultra instinct fanfiction, Izuku gains a new level of power and awareness. His movements become more instinctive and fluid. For example, he can dodge attacks without even thinking consciously. Also, his combat style often changes to be more aggressive yet precise.
Yes, he does. Vegeta achieves Ultra Instinct in the manga at a crucial point in the story.