You could write a fanfic where the monsters swap their habitats and have to adapt to new environments. For example, a water - based monster has to live on a mountain and learn to sing in a different way due to the change in acoustics. It would be a fun exploration of how the monsters interact with new surroundings and how their music changes as a result.
One idea could be a story where the monsters and aliens have to team up against an even greater threat from another dimension. Maybe a powerful interdimensional being that wants to conquer both Earth and the alien planets.
Perhaps a story where a young hero is on a quest to find the origin of the monsters in order to stop them from overrunning the world. They face many challenges and meet various strange creatures along the way. They might also discover that some of the monsters are not as evil as they seem and are actually being controlled by a greater evil force.
First, think about your favorite monsters. Maybe you like the cute PomPom or the spooky Entbrat. Then, come up with a simple plot like a monster getting lost and the others going on a search. Just start writing down your ideas.
One idea could be a monster girl who is a mermaid and she has to adapt to living on land in an everyday human world. Maybe she gets a job at an aquarium and has comical encounters with visitors who don't know she's a real mermaid. Another could be a harpy monster girl trying to fit into a human school, dealing with things like flying in gym class and eating different food from what she's used to in her nest.
One idea could be to create a story where the protagonist is a commoner who accidentally stumbles into the royal palace and catches the eye of the prince. The prince is initially intrigued by her non - royal ways and they start to develop a friendship that slowly turns into love. Another idea is to write about a modern - day girl who gets transported into the 'be my princess' world and has to navigate the complex rules of the court while also dealing with her growing feelings for one of the princes. A third idea could be a fanfic that focuses on the backstory of one of the princes, exploring his past traumas and how he overcomes them with the help of the main character.
One idea could be a story where Percy, the main character, discovers a new type of monster that has unique powers. He has to figure out how to communicate with it and whether it's a friend or foe. Another idea is a fanfic set in a different time period, like Percy in a medieval world full of monsters, and he has to use his skills in a whole new way. Maybe he becomes a knight who battles the most dangerous monsters.
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.
Another interesting fanfic idea would be to write an alternate ending. Maybe in your version, the characters find a different way to liberate themselves, not the way it was shown in the original. This could involve different relationships forming and different career choices for the characters.
One idea could be a crossover with the 'Harry Potter' universe. Imagine the My Hero Academia characters attending Hogwarts and having to adapt to magic while still using their quirks. Another could be a crossover with 'Star Wars', where the heroes of My Hero Academia find themselves in a galaxy far, far away and have to deal with the Force and lightsabers. Maybe a crossover with 'The Avengers' too, where they team up to fight a common super - villain that threatens both universes.
One idea could be a story about a character who discovers a hidden realm in the My Craft world filled with rare and magical creatures. They have to learn to communicate with these creatures to gain access to their powerful abilities and use them to save their village from an impending doom.