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.
One idea could be that they are paired for a difficult hero training mission. Tokoyami's dark powers and Tsuyu's unique abilities would have to work in harmony. Maybe they get trapped in an area where Tokoyami's darkness actually helps them hide from a powerful enemy, while Tsuyu uses her long tongue to reach for resources or communicate with allies outside the area.
Tokoyami is often a bit brooding and serious. In the fanfiction, he could open up more because of Tsuyu's laid - back and friendly nature. Tsuyu, on the other hand, could become more confident in her decision - making when she sees how Tokoyami is so decisive. For example, when facing a difficult choice during a battle, she might remember how Tokoyami would act.
The Ultra Instinct in a fanfiction might also have an impact on the character's growth. It could be a new milestone in their hero journey. Instead of just relying on their existing quirks, they have this new ability to master. This could lead to new relationships and rivalries. Other characters might be in awe or jealous of this new power, which would create more drama and tension within the story.
Tokoyami has a rather dark and mysterious aesthetic, which makes his stories seem more believable and spooky. His quirk, Dark Shadow, also gives him an affinity for the darker aspects of things.
Tokoyami might tell a story about a dark shadow creature that lurks in the abandoned parts of UA High. It could be a being that feeds on the fear of students who venture there alone at night.
In a fanfic scenario where Naruto consumes the Rumble Rumble fruit, it would be a huge game - changer for him. His chakra nature could potentially be affected, and he might be able to generate electricity from his own chakra. This new power could help him in battles against stronger opponents. He could send out electric waves to disrupt enemy attacks or even paralyze them. Moreover, it could also change his relationships with other characters as they would have to adapt to his new - found abilities.
Well, clearly Sweetballe and Rumble are the main characters. Sweetballe might be a pony with a kind heart and some special abilities or qualities. Rumble, as a pegasus, is likely to have flying skills and a personality that can either complement or contrast with Sweetballe's.