Maybe the OC is from a different island and Keoni helps her adjust to life on Moana's island. Along the way, they fall in love. There could be cultural clashes and misunderstandings at first, but as they get to know each other better, they bridge the gaps between their two cultures.
One idea could be an OC who is a long - lost relative of Moana. This OC could have been separated from the family during a great storm years ago. They could have unique skills, like being able to communicate with a different type of sea creature that Moana has never encountered. This would bring new adventures as Moana and the OC try to figure out how this new discovery could help their island.
Start with small interactions. For example, have them meet by chance on the beach and share a simple conversation about the beauty of the ocean. This initial connection can be the foundation for a deeper relationship.
Perhaps the male OC is a mysterious figure who appears to Moana in her dreams before she even sets out on her journey. He seems to know a great deal about the heart of Te Fiti and the approaching darkness. When they finally meet in person, he reveals that he has a connection to an ancient power that can either help or hinder Moana's mission, and she has to figure out how to work with him.
Another concept could be a crossover where some of Naruto's friends like Sasuke or Sakura also end up in Moana's world. They all form a team with Moana and her friends to fight against a dark force that is trying to take over the ocean and the land.
One common plot idea could be a crossover where characters from Coco end up in the world of Moana. For example, Miguel could meet Moana and they could go on an adventure together, perhaps using Miguel's music skills and Moana's sailing expertise to solve a new problem in Moana's world.
Another plot could revolve around an OC who is a Spartan trainee that doesn't quite fit the mold. They have a different way of thinking and approaching combat. In Halo 4, they end up being the key to solving a mystery related to the Prometheans. Maybe they find a way to communicate with the Prometheans and convince some of them to stop fighting the humans.
One idea could be an Otsutsuki OC who comes to Earth not for conquest but to study the unique life forms here. They could befriend some of the local ninjas and learn about the concept of friendship, which is alien to the Otsutsuki. This would lead to a series of adventures where they have to protect their new friends from other more hostile Otsutsuki members.
One idea could be exploring the origin of the OC in the covenant world. Maybe they have a special power that sets them apart from the regular covenant members.
One idea could be an OC who is a new Wolfblood in a pack and has to prove themselves. Maybe they have a unique power that the others don't have and they have to learn how to control it while fitting into the pack's hierarchy.
One idea could be an OC who is a long - lost relative of one of the main characters. They suddenly show up and cause a stir in the power dynamics. For example, if they are related to the main heir, they might have a claim to the inheritance as well, leading to all kinds of family drama and power struggles.