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.
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.
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.
Boo would probably interact with the baby monster in a very gentle and affectionate way. She might hug it a lot and try to communicate with it using her own unique baby language. Since she's a child herself, she would likely see the baby monster as a new playmate and would share her toys with it.
A possible plot could be that there's a baby monster that is scared of everything in Monsters Inc. Boo, who has experienced the monster world, takes it upon herself to show the baby that there's nothing to be afraid of. She introduces the baby to all the friendly monsters she knows. Sulley and Mike are a bit skeptical at first but soon see how Boo's kindness is helping the baby grow braver. As time goes on, the baby becomes more confident and starts to explore the world on its own with Boo by its side as a guide.
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.
Monsters Inc is not based on a real story. It's an imaginative concept where monsters live in a parallel world. The movie uses this fictional setting to tell a story about friendship, like the relationship between Sulley and Mike. They work in a company that has very unique and out - of - this - world rules.