A secret organization could be the antagonist. They might see Bella as a threat to their hidden agenda. For instance, if she has stumbled upon some information about their illegal activities, they lock her in a room to keep her silent and prevent her from sharing what she knows with the outside world.
Maybe it's a malevolent spirit or entity. In a fanfiction with supernatural elements, a dark spirit could lock Bella in the room as part of its plan to feed on her fear or use her for some other dark purpose.
Perhaps it's a more romantic take. Bella could be locked in a room by a love interest who is too shy or conflicted to face her directly. So, in a rather clumsy way, they lock her in a room with the intention of finally confessing their feelings but are struggling to find the right words.
It could be another vampire clan. Maybe they are jealous of the Cullens' interest in Bella or they think that Bella's presence among the Cullens will disrupt the balance of power among vampires. So they try to get Bella from the Cullens, making them the main antagonists.
A new teacher who is secretly a dark wizard could be the antagonist. They might have come to the school with the intention of stealing Twilight's powers. By locking her in the school, they can have more time to try and carry out their evil plan without interference from the outside world.
It could be a dark wizard or sorcerer who wants the dragon egg for its power. They might try to steal the egg from Bella.
It could be Danzo. He was always power - hungry and had his own ideas about how the village should be run. He might see Naruto as a threat to his plans, so he locks Naruto away as a baby. He wants to control the village without the interference of the jinchuriki's power.
😋I recommend "My Beautiful Demon Wife" to you. This novel belongs to the urban life category. When the female protagonist was young, her family prevented her from going to the earth temple in the back mountain. Only when she grew up did she realize that there was a woman there. Coincidentally, the novel also contained a plot where the female protagonist was afraid of dogs, because she had once been chased by a dog until she was out of breath. I hope you like this fairy's recommendation. Muah ~😗
It could be a vengeful Death Eater. Maybe Draco once crossed this Death Eater in some way, and now the Death Eater wants to teach him a lesson by locking him in a cage. This Death Eater might be jealous of Draco's status in the Malfoy family or his closeness to Voldemort at one time.
She might meet Aro, one of the leaders of the Volturi.
She might meet her younger self. It would be an odd yet fascinating encounter, as she could see how she used to be and compare it to who she is now.
There is also a possibility that a lone vampire, who has a sense of justice or who has been secretly observing Bella and has grown fond of her, decides to take on the Volturi and rescue her. This vampire could have some special abilities or knowledge that gives them an edge against the powerful Volturi.