It could be that she has some special power she's been hiding all along. Maybe she can communicate with animals secretly and she's been too afraid to tell the Cullens before.
The Cullens might be shocked at first. If Bella's secret is something really unexpected like she has a hidden power, they will need some time to process it. They could be initially speechless.
Edward is a likely candidate. Since he is so attuned to Bella, he might pick up on some strange behavior or scent that leads him to suspect she is hiding something.
Maybe it could be about Bella Swan having superpowers and joining the Avengers. For example, she could have a power related to her ability to resist vampire powers in 'Twilight' and use it to help the Avengers in their battles.
One possible plot is that Bella discovers a hidden dance community while being a secret dancer in the fanfiction. She gets involved with this community and through her dancing, she starts to find herself and gain confidence. But she has to keep it a secret because of the rules of this exclusive community.
Well, it could be due to the way the Cullens interact with the other people in Forks. If Bella notices that they are being manipulative or using their powers in a way that she doesn't like to influence the townspeople, that could make her start to hate them. Also, if there is some kind of power struggle within the Cullen family that starts to affect Bella negatively, like she gets caught in the middle of it or she sees how it's causing harm to those she cares about, she might turn against them.
She might start by giving them the silent treatment. Just ignoring them completely when they try to talk to her, which would be a very human way of showing anger.
One possible reason could be that the Cullens make a decision about her future without consulting her. For example, if they decide she should leave Forks for her own safety without asking her how she feels about it.
There could be a dance instructor. Maybe someone who notices Bella's potential even though she's trying to keep her dancing a secret. This instructor could play a key role in her journey as a dancer.
In the 'the cullens kidnap bella fanfiction', it could be that the Cullens have some hidden motives. They might believe that Bella has a special power or connection that they need to study or keep safe. So they take her against her will, which then leads to a complex relationship dynamic between Bella and the Cullens as she tries to figure out what they really want.
In the 'bella is kidnapped by the cullens fanfiction', one possibility is that Bella is kidnapped to be part of some vampire ritual. However, she refuses to cooperate at first. The Cullens then have to try and convince her that it's not as bad as it seems. This could involve showing her their more human - like qualities, like their love for art and music. As Bella spends more time with them, she may start to see them in a different light and even develop feelings for one of them.