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.
Perhaps the fanfic could be about the monsters going through a mid - life crisis. They start to question their purpose in scaring kids and think about other ways to contribute to the energy collection. This could lead to some comical and heart - warming moments as they try to find new meaning in their lives.
In an all - grown - up Monsters Inc fanfic, the world itself could have evolved. There could be new technologies for scaring or collecting energy. The characters might have to adapt to these changes. And if Boo is all grown up and involved, she could bring a human perspective that helps the monsters improve their scaring business in unexpected ways. Maybe she could introduce new concepts from the human world that the monsters had never thought of, like different ways of using sounds or lights for scaring.
It could be about Boo using her experiences in the monster world to make a difference in the human world. Maybe she becomes an advocate for better understanding between humans and monsters.
By showing how her time in the monster world influenced her values. For example, if she learned about kindness from Sully, as an adult she could be extra empathetic.
One idea could be that Anya and Damian grow up and both enter the world of international diplomacy. Anya, with her telepathic abilities still somewhat intact, has an edge in understanding the true intentions of other diplomats. Damian, having grown into a confident and intelligent young man, works alongside Anya to solve international crises. They start off as rivals in this new world but gradually realize their feelings for each other as they face challenges together.
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.
They could fit in by using their skills. The keyblade wielders could protect the monsters from any real threats. And the Monsters Inc crew could teach the KH characters about their own form of energy, screams. It's a simple exchange of what they are good at.
Well, they could have a really wild adventure. Maybe the KH characters would be confused at first in the world of Monsters Inc. Sora might try to fight the monsters thinking they are Heartless. But then they could realize that the monsters are just trying to do their jobs. Kairi could make friends with Boo easily, and Riku might be intrigued by the technology in the monster world.
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 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.
Charlotte's family members could also be main characters. Her parents, for instance, who have influenced her upbringing and continue to be part of her life as she grows. They might be shown guiding her through different life decisions or having their own story arcs that intersect with Charlotte's.