Another plot could involve a Monster Girl Quest within the Forbidden Forest. The characters discover a hidden community of monster girls there. They have to earn their trust and complete a series of tasks assigned by the elders of the community. These tasks could be related to solving ancient magical mysteries that are tied to the history of Hogwarts and the wider wizarding world. It could lead to a new understanding between humans and these magical beings.
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.
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 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.
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.
Perhaps a group of wizards and witches form an alliance with Monster Hunters to stop a powerful dark magic - infused monster that is terrorizing both the wizarding world and the Monster Hunter's territory. They have to exchange knowledge, like the wizards teaching the Monster Hunters about potions for healing and the Monster Hunters showing the wizards how to use their unique weapons more effectively.
A plot could revolve around a monster girl who is trying to prove herself in the world of Monster Hunter. She befriends a human hunter, and together they face various challenges in the wild, like uncovering ancient secrets that are tied to both monster and human history. This could involve exploring forgotten ruins and deciphering old texts.
One idea could be that as a girl, Harry has a different approach to dealing with the challenges at Hogwarts. For example, she might use her charm more subtly to influence others and gain allies. Another could be that her relationships with male characters like Ron and Hermione develop in new and unexpected ways, perhaps with a more sisterly bond with Ron and a different kind of friendship with Hermione.
One idea could be that Harry discovers a hidden part of the wizarding world where there are 'monster girls'. Maybe he stumbles upon a secret club at Hogwarts. Through his interactions with them, he learns about their unique cultures and magic. This could lead to him wanting to be more involved with their world, perhaps even using his skills to help them with problems they face.
One idea could be that Harry's transformation into a monster girl makes him/her (depending on how the transformation affects gender identity) discover a whole new world of magical creatures that were previously hidden from normal wizards. He/She could form unique bonds with these creatures and use their newfound abilities to solve mysteries in the wizarding world.
Harry Potter himself could be the main character. He has a history of taking on difficult quests and helping those in need. His sense of justice would drive him to help the monster girls on their journey. He could use his skills in Defense Against the Dark Arts and his knowledge of the wizarding world to overcome obstacles.