Definitely Naruto and Star Butterfly. Naruto is a powerful and determined ninja, and Star is a magical princess. Their different powers and personalities would create a lot of interesting interactions. For example, Naruto's never - give - up attitude could inspire Star when she faces a tough situation in her own world.
Sasuke from Naruto could also be a great focus. He has his own complex backstory and powers. Paired with someone like Marco from SVTFOE, who is also a bit of an outcast at times, they could have some deep conversations and adventures. They could both learn from each other's experiences and grow as characters.
Hinata from Naruto and Pony Head from SVTFOE could make for an interesting pair. Hinata is shy but strong, while Pony Head is loud and brash. Their interactions would be full of humor and they could also help each other in unexpected ways, like Hinata teaching Pony Head some self - defense moves and Pony Head helping Hinata be more assertive.
Naruto himself is an obvious choice. Since it's 'Naruto Book 1', there's a lot of room to expand on his character development at the very beginning. Also, Sasuke is a great character to focus on. You can explore his initial aloofness and how it might be different in a fanfiction, perhaps showing more of his inner thoughts and feelings at that early stage.
Sasuke can be a great focus in dark Naruto fanfiction. His already brooding and vengeful nature can be taken to an extreme. For example, in some fanfics, he completely succumbs to the darkness within him, becoming a dictator - like figure in the shinobi world, ruling with an iron fist.
Shikamaru is a great character to focus on. His high intelligence makes for interesting plotlines. He can strategize his way out of difficult situations. For instance, in a fanfiction, he could be the key to solving a mystery that involves the entire ninja world.
Naruto himself is always a great focus. His journey from an outcast to a hero is full of potential for more drama. You can explore his inner thoughts and feelings more deeply, especially in situations not covered in the original story.
Sasuke is also a great choice. His relationship with Naruto is always a central point, and during the blank period, we can see how he's adjusting to life back in the village or if he's still on his own path of redemption. And his unique powers can bring a lot of action to the story.
For Kekkai Sensen, Leo is an interesting character to focus on. His abilities and his role in the story of Kekkai Sensen could be explored further in a fanfiction with Naruto elements. He could have some really cool interactions with Naruto characters.
The main family members can be great to focus on. Their relationships, power struggles, and dreams offer a lot of material. For example, the patriarch of the family who has to make tough decisions to keep the ranch going. Another good character could be the strong - willed matriarch who holds the family together. Also, the young and rebellious heir who wants to do things his own way.
Sasuke would also be great. His brooding nature could add an interesting contrast to the more upbeat and fun atmosphere of a high school musical. There could be a story about him opening up and showing his more creative side through the musical.
Sersi is a great character to focus on. She has a strong connection to humanity and her powers are quite interesting. You could explore her relationships with humans in more detail. Another good one is Ikaris. His loyalty and the internal conflict he might face are good material for a fanfiction. Then there's Kingo, with his love for movies and his unique charm among the Eternals.
All of the members of 4 town are great to focus on in different ways. Tae Young, for example, might be the subject of a fanfiction where his amazing dancing skills are the center of the story. Maybe he gets an offer to be in a big dance competition outside of the band, and he has to decide whether to take it or stay loyal to 4 town. Each member brings their own unique traits that can be explored in fanfiction.