Another plot twist could be that the transformation is part of a prophecy. The Slytherin boys turned girls are now key to solving a big mystery in the wizarding world. They have to learn to work together in their new forms and use their new - often underestimated - female perspectives to figure out clues and save the day.
Maybe the potion was created by a rival house as a prank, but it backfires in some way. The transformed Slytherin boys - now girls - end up being more popular or more successful in certain aspects, causing jealousy among the other houses. And then there could be a race to find the antidote before things get too out of hand.
A good way to find such a fanfic could be to start with your favorite Harry Potter fanfic authors. See if they have any works that are related to Slytherin characters and magical transformations. Even if they don't have an exact match, they might know other authors who write this kind of story.
Another plot twist might be that the person who gave Marsha the love potion was actually in love with her themselves. They thought that by making her fall in love with someone else through the potion, they could be closer to her and perhaps win her heart in the end. So, they watch in agony as Marsha falls for another, all the while planning to reveal their true feelings at the right moment.
One possible plot twist could be that they go into Slytherin and are initially shunned by the other Slytherins. But then they discover a hidden group within Slytherin that has different values from the traditional Slytherin ones. This group is more inclusive and values knowledge and friendship over pure - blood supremacy. Harry and Hermione then work with this group to change the perception of Slytherin within Hogwarts.
One possible plot twist could be that the girl who is pregnant has some sort of magical ability related to the pregnancy that no one expected. Maybe she can communicate with the unborn child in a magical way. Another could be that Harry discovers he's not the real father, but due to some magical mix - up, he has to take on the role anyway.
Well, perhaps the 111 boys and some of the 125 girls are in a competition. It's a talent show. Each of them has different talents. Some boys are good at sports - based acts, while the girls shine in art - related performances. In the end, they all learn to appreciate each other's skills and come together as friends.
In this fanfiction, a great plot twist could be that the Slytherin character has a hidden identity as a member of an ancient magic - protecting group. The betrothal to Harry is part of a bigger plan to unite the houses against a new, emerging dark force. As they come together, they face a lot of opposition from both sides, Gryffindor and Slytherin, but through their relationship, they start to bridge the gap between the two houses.
One possible plot twist could be that the character who smirks is actually hiding a deep secret. Maybe they are planning something devious against the main character. For example, in a mystery - themed fanfic, the smirking character could be the mastermind behind a series of strange events that the other characters are trying to solve.
The plot might develop as the girls realize that their transformation into origamis is part of a bigger plan by an evil force. The evil force wants to use their new forms to control the balance between the magical world and the normal world. So, the Precure girls have to fight against this while still getting used to their origami bodies. They could learn special origami - based spells that are only accessible in their transformed states, and they must master these spells to defeat the evil and break free from the unwanted transformation.
One possible plotline could be that Naruto, as a smart female, uses her newfound intelligence to outwit the Akatsuki. She could come up with brilliant strategies to protect the village and its people, something that might surprise her old friends at first. Another plot could involve her using her intelligence to study and master new jutsus that were previously difficult for Naruto as a male. Maybe she could also use her charm and intelligence to negotiate better treaties between the Hidden Villages.
A really unexpected plot twist could be that the girls - Slytherins now have special powers that come with their new form. For example, they might be more attuned to nature magic or have enhanced intuition. This could change the power dynamics within Hogwarts as they are no longer just seen as the traditional Slytherins, but as a new and unique group with their own abilities.