Definitely Rainbow Dash. Since Scootaloo looks up to her, Rainbow Dash could offer some flying tips or be a mentor figure. Fluttershy would also be great. Her gentle nature could contrast with the more energetic Scootaloo and Rumble, and she could help them when they encounter some scared animals during their adventures.
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.
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.
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.
Deku would be a great character to pair with Obito. Deku's pure - hearted nature and his drive to be a hero could contrast with Obito's more complex and troubled past. Obito could see in Deku the kind of hero he once wanted to be, and he could help Deku in ways that All Might or other heroes couldn't, especially when it comes to dealing with darker aspects of the hero world.
Obviously, Eraserhead and Ms. Joke are the main ones. But you could also include their students. For instance, having some of Eraserhead's Class 1 - A students interact with Ms. Joke could add some interesting dynamics. Maybe Midoriya could be curious about their relationship.
The Litten line (Incineroar) is a great starter for the Ultra Moon story. Incineroar is a strong Fire - Dark type. It has high attack power and can learn some really useful moves as it levels up. It can hold its own in many battles and is quite intimidating to opponents.
A good team could be Incineroar, Primarina, Lycanroc, Vikavolt, Mimikyu and Alolan Ninetales. Incineroar is strong and has great offensive capabilities. Primarina has good special attacks. Lycanroc is fast and can deal decent damage. Vikavolt has high - powered electric moves. Mimikyu can be a tricky opponent for foes, and Alolan Ninetales is both beautiful and powerful with ice - type moves.