An OC could be a monster that was once a human. They got transformed into a monster due to some strange accident in the monster world. Their struggle to adapt to being a monster while still having human memories and emotions could make for a great fanfiction. They could try to change the way Monsters Inc operates from the inside.
One idea could be an OC who is a new type of monster with a unique ability, like the power to change colors to blend in different sceneries in the human world. This would add an interesting twist to the story as they could use it for both scaring and befriending humans.
Perhaps an OC that is a very small monster but has a big heart. They might not be good at scaring like the others, but they are excellent at making friends with other monsters. They could start a new trend in Monsters Inc where friendship is more important than scaring power, which would cause a lot of changes in the company.
One idea could be exploring the backstories of the monsters more deeply. For example, what was Sulley's life like as a young monster? Another could be a story where Boo somehow returns to the monster world as an adult. Or a fanfiction about the monsters having to deal with a new kind of energy source other than screams.
One idea could be a crossover with 'Toy Story'. The monsters could accidentally enter Andy's room and interact with the toys. It would be hilarious to see Sulley and Woody trying to understand each other's worlds.
One idea could be creating an OC (original character) who is a misfit among the traditional monster students. For example, a small and timid monster in a school full of big, scary ones. The story could revolve around this OC trying to find their place and prove themselves in the university.
There could be a fanfiction where a human scientist figures out a way to communicate with the monsters in Monstropolis. This scientist then tries to bridge the gap between the two worlds, leading to a series of events where both humans and monsters learn more about each other. For example, the scientist might introduce the concept of different human cultures to the monsters, which they find fascinating.
First, think about your OC's appearance. Are they big or small? Do they have fur or scales? Then, consider their personality. Are they shy or outgoing? After that, come up with a plot. It could be something like your OC trying to break a record in scaring at Monsters Inc. Just start writing down your ideas and let the story flow.
Another idea is that all the characters are now parents themselves. They have to balance their work at Monsters Inc with raising their own little monsters. It could show how they use their experiences as scared kids or scarers to be better parents.
One of the scariest stories could be about the time when a particularly large and menacing monster accidentally entered the human world through a malfunctioning door. The chaos it caused as humans ran in panic was terrifying. It was a huge risk not only for the humans but also for the secrecy of the Monsters Inc world.
Monsters Inc. is set in a world where monsters generate energy from children's screams. But as the story progresses, we discover that laughter can generate even more powerful energy, leading to a major shift in the monsters' way of doing things.
Well, you could write a fanfic where the Adventure Inc team gets separated in a large, maze - like underground cavern. Each member has to use their individual skills to find their way back to the others while also uncovering secrets of the place. This would add a lot of tension and character development as they face their own challenges alone.
One plot idea could be a new super monster arriving at the school. This new monster has a unique power that the others have never seen before. It could create some chaos at first as the other super monsters try to figure out how to get along with this new addition. Another idea is a time - traveling adventure where the super monsters go back in time to meet their ancestors and learn about their own powers' origins. And a third idea might be a mystery where the super monsters' powers start to disappear one by one, and they have to work together to find out what's causing it.