Well, a possible plot for a Ben 10 Monster Girl Quest fanfic could be that an ancient artifact starts turning the girls in Ben's life into monster girls. Ben initially struggles to understand them as their forms are so different. But as he spends time with them, he discovers that they have unique abilities that can help in his fight against a new alien threat. For example, a girl turned into a fire - based monster girl could help him against ice - based aliens. He then embarks on a quest to find a way to reverse the transformation while also using their powers to protect the planet.
One idea could be that Ben Tennyson finds himself in a parallel dimension where the Omnitrix has transformed all the female characters into monster girls. He has to figure out how to change them back while also dealing with the new powers and personalities of these monster girls. Another plot could involve a group of evil forces trying to use the monster girl situation to take over the world, and Ben has to team up with some of the friendly monster girls to stop them.
One idea could be a quest where the main character has to rescue a monster girl who is being wrongly persecuted in the wizarding world. Maybe she has a unique power that the dark wizards want to use for evil, and the protagonist, inspired by Harry Potter's courage, sets out to protect her.
One idea could be that as a girl, Ben has to face different social challenges while still using the Omnitrix to save the day. For example, some might underestimate her because of her gender, but she proves them wrong with her heroic deeds.
One idea could be a quest where the main character has to rescue a monster girl who has unique magical abilities that are crucial to the wizarding world. Maybe she can communicate with magical creatures in a way no one else can, and dark forces want to use her for evil. So the protagonist teams up with his Hogwarts friends to find and protect her.
Another great storyline could involve a slime girl in the Monster Girl Quest fanfic. Maybe she discovers a unique power within herself that allows her to transform into different forms, and she uses this power to help the other monster girls and humans co - exist peacefully. For example, she could transform into a bridge to help humans cross a dangerous ravine in the monster - dominated land.
One idea could be to create an OC who is a half - human, half - monster girl trying to find her place in a world that fears her kind. She could go on a journey to prove that she's not a threat.
One idea could be a crossover with a popular fantasy RPG. The protagonist from Monster Girl Quest could find themselves in a world filled with different races and magic systems from the other game. They'd have to adapt and maybe form alliances with new characters. For example, if it was a crossover with 'The Elder Scrolls', the monster girls could interact with the various guilds in Skyrim, like the Thieves Guild or the College of Winterhold.
It could be Vilgax. He might see the chaos caused by the monster girl transformations as an opportunity to finally defeat Ben. He could try to use the monster girls' new powers against Ben or even find a way to control them to do his bidding.
A plot could revolve around a monster girl gazer who accidentally gets a special power that allows him to understand the languages of all monster girls. This leads to him becoming a mediator between different monster girl tribes that are at war. He uses his newfound ability to broker peace and in the process, falls in love with a particular monster girl from one of the tribes.
One idea could be a story where a young adventurer stumbles upon the world of the Monster Girl Matango. He has to navigate through their society, learning about their unique customs and powers. Maybe he forms an unlikely friendship with a Matango girl who helps him understand their world better.
One idea could be a story where a young adventurer stumbles upon a hidden forest inhabited by monster girls and fairies. The adventurer has to learn to co - exist with them and discovers that the monster girls and fairies have their own complex society and unique powers. For example, a mermaid - like monster girl who can control water helps the adventurer cross a dangerous river in the forest.