One interesting plot twist could be that the Beast is actually under a double curse. Not only is he a beast, but he also has a memory curse that makes him forget his true self. When Belle discovers this, she has to find a way to break both curses.
One interesting plot twist could be that Belle actually has a hidden power that she discovers while in the Beast's castle. This power could be related to time-travel, which ties into the 'Back in Time' aspect. Maybe she can manipulate time to prevent certain events from happening in the past, like the Beast being cursed in the first place.
Perhaps a plot twist is that the Beauty is actually a parahuman with a power that she doesn't fully understand until she meets the Beast. Their connection then unlocks a new aspect of her power, changing the dynamic between them and their goals. For example, her power could be related to empathy, and the Beast's presence intensifies it to the point where she can influence the emotions of other parahumans around her.
A great plot twist could be that the 'beast' is actually a dark wizard in disguise. He pretends to be cursed to lure in the 'beauty' who is a powerful young witch. But as the story progresses, she discovers his true identity and has to use her magic to defeat him instead of breaking a curse with love.
One interesting plot twist could be that the Beast is actually a cursed prince from a rival kingdom, and the curse was placed by an evil sorcerer who wanted to start a war between their kingdoms. Another could be that Beauty has a hidden power that she discovers while in the castle, and this power is the key to breaking the curse in a completely unexpected way. Maybe she can communicate with the enchanted objects in a way that no one else can, and they reveal a secret about the curse.
Another plot twist might be that the Beast is not actually a 'beast' in the traditional sense. In the fanfiction, he could be a wizard under a false appearance curse, and Hermione discovers this. It changes the whole dynamic of their relationship because now they are both magic - users, and she has to figure out how to break the false - appearance curse rather than the typical Beauty and the Beast curse.
A great plot twist is when the objects in the castle, instead of just being there to serve as comical relief or side characters, are actually the ones who put the curse on the Beast in the first place, out of some misdirected sense of loyalty or protection. Also, Beauty could turn out to be a long - lost princess from a neighboring kingdom, and her union with the Beast has political implications that add a whole new layer to the story.
In some Beauty and the Beast pregnant fanfictions, a plot twist could be that the source of the Beast's curse is actually related to a prophecy about his future offspring. So, when Belle becomes pregnant, it sets off a chain of events that were foretold long ago. This could involve other magical beings coming to the castle to either protect or harm the baby. Another plot twist might be that Belle's pregnancy gives her the ability to communicate with the forest animals in a more profound way than before. They then become her allies in protecting the baby from any threats that may come from the dark corners of the enchanted forest.
One idea could be exploring the Beast's past more deeply. Maybe a story about his life before the curse, his relationship with his family and how it all led to his current situation.
One idea could be that Belle discovers a hidden garden in the Beast's castle that has magical plants which can talk and offer her advice. Another could be a backstory for the Beast, like how he got cursed in a more detailed and different way than in the original story. For example, he was tricked by a jealous wizard who wanted the castle for himself.
One interesting plot twist could be that just as Phineas and Isabella are about to confess their love, an old friend from Isabella's past returns. This friend has a secret crush on Isabella and tries to come between them. Another twist might be that Phineas suddenly loses his memory of his relationship with Isabella. Isabella has to work hard to make him fall in love with her all over again. It makes for a very emotional and engaging story.