One idea could be that the OC is a new vampire in town and Mitchell takes it upon himself to mentor the OC. They could get into all sorts of adventures, like dealing with vampire hunters together.
The OC could be a witch who accidentally casts a spell on Mitchell. This leads to a series of comical and dangerous situations as they try to reverse the spell while also getting to know each other better and forming an unlikely friendship.
Maybe the OC is a werewolf who has a strange connection with Mitchell. Their relationship could be explored as they both try to navigate the challenges of being non - human in a human world. For example, they might team up to protect a group of humans from a common enemy.
An OC who is a human detective investigating strange occurrences in the 'Highschool DxD' world. They could end up forming an unlikely alliance with the devils. This would bring a new perspective as they try to understand the complex relationships and power dynamics in the series.
One idea could be an OC human who discovers a hidden power within themselves that can interact with the Cybertronian technology in unique ways. For example, they might be able to heal damaged Transformers using a strange energy they possess. Another plot could involve an OC human getting caught in the middle of a battle between the Autobots and Decepticons and having to use their wits and new - found friendships with the Autobots to survive and help them. A third idea is an OC human who is a mechanic and accidentally creates a new device that attracts the attention of both Autobots and Decepticons, leading to a series of adventures as they try to keep it out of the wrong hands.
A good plot could involve a human OC who is a doctor. They end up treating a ghoul who was severely injured in a fight. This starts a strange relationship where they try to understand the nature of ghouls better. As they get more involved, they learn about a secret organization within the ghoul world that is planning something big that could endanger all of Tokyo. The OC then has to decide whether to stay out of it or try to stop it. It's a plot full of moral dilemmas and action. Also, a plot where the human OC is an artist and they use their art to show the hidden side of the ghoul - human relationship could be interesting. Their art could either be a way to promote peace or it could stir up more trouble depending on how different groups react to it.
Maybe the Vulcan OC is on a mission to study human culture and meets the human OC. At first, the Vulcan finds the human's behavior very illogical. But as they interact more, like when they attend a human music concert together, the Vulcan starts to understand the beauty in human emotions and they fall in love.
Another idea is a human OC who is a detective in Beach City. They start to notice strange happenings that are related to Gem activity. As they dig deeper, they get involved with the Crystal Gems and become an ally. They could use their detective skills to help the Gems solve mysteries, like finding missing Gem shards or uncovering the secrets of an ancient Gem structure.
An interesting concept could be a male human OC who is a talented artist. He gets inspired by the colors and beauty of the MLP world and starts creating art that has a magical effect on the ponies. His art could help solve problems in Equestria, like bringing back the lost colors of a fading meadow.
The human OC could be a relative of an important figure in the vampire - hunter world. Kaname wants to use her to gain an advantage in the power struggle, while Zero wants to protect her from being used. The OC, in the meantime, is trying to figure out her own place in this strange world. She might start to develop feelings for both Kaname and Zero. As the story unfolds, she discovers secrets about her family that tie her even closer to the conflict. Kaname and Zero then have to decide whether their feelings for the OC are more important than their long - standing enmity.
Perhaps an OC is a young archaeology student who idolizes Nathan Drake. They manage to get involved in one of his expeditions through sheer determination. Along the way, they learn from Drake and also teach him new things about modern archaeological research methods. It could be a story of growth and adventure for both characters.
One idea could be an OC who is a lightning - style user in Kumogakure and has to face a powerful enemy from another village. The OC could discover a new jutsu during the battle.
A story about an OC who is a healer in the Injustice 2 world could be really engaging. In a world full of battles and destruction, this OC could be trying to heal not only the physical wounds but also the emotional and psychological damage that the characters have endured. They might face challenges like being kidnapped by villains who want to use their healing powers for their own evil purposes.