One idea could be an OC who is a half - angel, half - demon and is on a quest to find their true identity. They struggle between the good and evil tendencies within them.
Perhaps an OC that is a demon but has a change of heart after falling in love with an angel. Their relationship could be the central focus, with all the challenges of being from opposite sides in the eternal battle.
One idea could be the protagonist discovers a hidden power within themselves while being held captive by the demons. This power could be the key to their escape or a way to turn the demons' own plans against them.
One idea could be the demon's mate having a special power that is the key to saving the demon realm from an ancient curse. Another could be the struggle of the mate to adapt to the demon's dark and dangerous world while still holding onto their own humanity. Maybe the demon's mate is from a different supernatural species, like an angel, and their relationship is a forbidden love that defies the rules of both their worlds.
How about a crossover with a post - apocalyptic world? The Angels of Death duo would find themselves in a world full of ruins and scarce resources. They would have to deal with new threats like mutated creatures and other desperate survivors. Their relationship could be further tested in this harsh new environment.
Another plot could be a what - if scenario. What if Zack and Rachel escaped the building and had to live in the outside world? They would face new challenges like dealing with society's norms and trying to hide their past. It could show a different side of their relationship as they adapt to a new environment.
In some fanfictions, Dean might raise angels with a special purpose. Like he raises a group of angels to help him save Sam who has been trapped in a different dimension or something equally dire. The angels have their own motives and secrets, and as they work with Dean, all these secrets start to unfold, leading to a complex and engaging story.
A plot could revolve around a new villain emerging from the Land of Demons. This villain has a special connection to Naruto's past, and Naruto must face his own inner demons as well as the external threat to save the Land of Demons and his own world. The story could explore themes of redemption and self - discovery.
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 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.
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.