An interesting MCU OC fanfic idea could be an OC who is a journalist. They are always on the hunt for the next big story about the superheroes. This could lead them into all sorts of dangerous situations as they try to get exclusive interviews or expose secrets. Another could be an OC who is a mutant in the MCU (even though mutants are not as prominent yet). This OC would have to deal with being different and feared, while also trying to use their powers for good. And a third idea is an OC who is a reformed villain. They were once on the wrong side but now want to make amends and help the heroes in their fight.
One idea could be creating a male OC who is a brilliant scientist from a small town. He discovers a new form of energy that attracts the attention of Tony Stark. Another could be a male OC with super speed who has to hide his powers until he's forced to use them to save a city. Maybe a male OC who is an alien prince stranded on Earth and has to learn about human culture while also trying to find a way back home.
One idea could be that the male OC is a new superhero who saves the day in unique ways. His harem could consist of different female characters from the MCU, like a SHIELD agent, an Asgardian, and a Wakandan warrior. They are all attracted to his courage and his different perspective on heroism.
One idea could be that Clint and Natasha are transported into a fanfic where they have to face alternate versions of their MCU villains. They could use their unique skills and teamwork to outwit these foes. For example, facing a more powerful Loki or a group of Hydra agents with enhanced abilities. Another plot could involve them getting trapped in a fanfic where they are in a romantic comedy setting within the MCU universe, which would be a fun change of pace as they try to figure out how to get back to their normal world while dealing with all the comical situations.
One interesting MCU OC fanfiction could be about an OC with the power to manipulate time who teams up with the Avengers during the Endgame battle. This OC could use their power to reverse some of Thanos' actions in a unique way.
A Sonic OC fanfic could center around an OC who is a healer. In a world full of battles against Dr. Eggman, this OC would use their healing powers to patch up Sonic and the gang after tough fights. They could also have a backstory about how they learned their healing abilities, maybe from an ancient and forgotten tribe in the Sonic world. This would add depth to the character and the fanfic as a whole.
You could write an OC fanfic where the OC is a female assassin. She has a rivalry with Agent 47 at first, thinking he is taking all the good contracts. But as the story progresses, they end up having to team up to take down a powerful criminal organization that is targeting assassins. Her special skill could be hacking into high - security systems to gain access to targets.
A Hiccup OC fanfic could also explore what would happen if Hiccup's OC was from a different tribe. They could have different traditions and skills, and the story could focus on how Hiccup and his friends learn from this new character. For example, the OC could teach them a unique way of dragon training that they've never seen before.
Another interesting concept is a Kryptonian OC who is part of a secret society on Earth. They keep their identity hidden from the world, but within this society, they use their powers to maintain order and justice in a more shadowy way. They might have to deal with internal power struggles within the society as well as external threats to their secrecy.
You could have an OC who is a shy artist fairy. In the fanfic, Tinkerbell discovers the OC's amazing talent for painting the most beautiful landscapes of Neverland. They then team up to create a huge mural in Pixie Hollow that showcases all the different areas of their magical world. The OC might start off really timid but gain confidence with Tinkerbell's encouragement.
One idea could be creating an OC that is a hybrid animal, like a fox - rabbit mix, and exploring how they fit into Zootopia's society. They might face unique challenges and prejudices.