A human character who has always felt like an outcast in the Human Realm could be the main character. This person discovers the magic of the Owl House and realizes that they finally belong somewhere. They could be the bridge between the human world and the magical world of the Boiling Isles. They might start to learn magic and use it to solve problems in both realms, all while trying to keep the two worlds from colliding in a disastrous way.
It could be King. Imagine King in the Human Realm, thinking he's the king of everything there too. He could get into all sorts of comical situations as he tries to assert his dominance. Or perhaps Eda, who might be in the Human Realm looking for a particular ingredient for a potion that can only be found there. She would bring her sassy and powerful self to the human world and have to deal with the mundanity of human life while also trying to achieve her goal.
Luz would be an obvious choice. Since she has a connection to both the Human Realm and the Boiling Isles, she could be in the Human Realm for some reason, like bringing a bit of the magic she's learned back home and facing new challenges there. Another option could be a new human character that gets involved with the magic of the Owl House, maybe a neighbor of Luz who notices strange things going on at her house and gets pulled into the adventure.
The human realm can be a source of contrast. For example, it's a place without the obvious magic seen in the Boiling Isles. Fanfictions might use this to create situations where characters from the Boiling Isles are shocked or fascinated by the human realm's technology or different social structures.
One interesting fanfiction could be about a human character from the human realm accidentally stumbling into the Boiling Isles and having to adapt. They might encounter strange creatures and magic, and their journey of discovery would be really engaging.
An older man who has lost his passion for life could be the main character. When he enters the Duel Monsters realm, he rediscovers his purpose and uses his life experiences, along with new - found magic within the realm, to become a hero in this strange new world.
One idea could be a human character from the Human Realm accidentally discovers a portal to the Boiling Isles and has to adapt to the strange new world while also trying to find a way back home. Another could be a story where a group of human friends get caught up in the magic of the Boiling Isles after a spell goes wrong in the Human Realm. Or perhaps a human with latent magical abilities is drawn to the Owl House in the Human Realm and begins to learn about their true powers.
Luz is an obvious choice. She can bring her knowledge of the Boiling Isles to the human world and show other humans the wonders of magic. Her enthusiasm and kind heart would make for great interactions with other human characters.
Maybe a kind-hearted storyteller. They have the ability to communicate with the animals in the Lion King world through their stories. The animals are fascinated by the human's tales and the storyteller uses this to bring peace and unity among different animal groups, especially during times of conflict. They could also use their stories to inspire Simba to be a better leader.
Luz is surely one of the main characters. She is often at the center of the action.
It could be a simple village girl. She might be kind, with a love for nature, which catches Legolas' attention. Maybe she has a special talent like being able to communicate with animals, which is something Legolas admires.
An orphan with special abilities could be the main character. They have no family or home, so when they land on Yautja Prime, they see it as a new start. Their special abilities, like being able to communicate with animals, might give them an edge in dealing with the native creatures of the planet and also intrigue the Yautja.
It could be an ordinary person, like a young adult who is just going about their daily life until something strange happens and they become a predator. This character would then have to learn how to deal with their new situation.