In a lot of fanfiction, Bella does end up with Jacob. Fanfic authors often explore different character arcs for them. Maybe in these stories, Jacob is more in tune with Bella's needs and desires. He might be more understanding and patient with her. The fanfiction could also change the dynamics of the other characters around them, like making Edward less of an obstacle. This allows Bella and Jacob to grow closer, and eventually, they end up together.
Sure, in certain fanfictions Bella ends up with Jacob. Sometimes, the fanfic writers rewrite the whole Twilight plot, making Jacob a more appealing character for Bella from the start. They might have Bella and Jacob meet under different circumstances that lead to a stronger bond between them right away. It could be something like Jacob saving Bella in a different way that makes her see him in a new light, and as the story progresses, their relationship blossoms into a romantic one.
One possible scenario is that Jacob reveals a secret that Bella doesn't like. This could make her feel betrayed and start to hate him.
One possible scenario is when the imprint causes Jacob to be very protective of the person he imprints on. Bella might see this as Jacob neglecting their previous friendship or bond. Another scenario could be that the person Jacob imprints on has a negative attitude towards Bella, and Jacob seems to side with the imprinted one, making Bella hate the whole situation.
Another scenario could be that Bella does something that endangers Jacob's family or the werewolf pack. This would make Jacob see her as a liability and develop hatred towards her. In some fanfictions, she might accidentally expose the werewolves to their enemies because of her association with the vampires.
In some fanfictions, they might kiss after a long period of misunderstanding. Jacob has been misinterpreting Bella's actions and vice versa. But when the truth finally comes out, they kiss as a way of sealing their new - found understanding and connection.
In some fanfictions, Jacob could get hurt while exploring an unknown area. There could be some sort of hidden danger, like a trap set by an enemy. Or perhaps he gets hurt in a natural disaster that occurs while he's in his wolf form and unable to fully protect himself. For example, a sudden landslide or a forest fire.
In 'bella gets hurt fanfiction', one possible scenario could be that Bella gets physically injured during a fight with a supernatural being, like a vampire. Another could be she gets hurt accidentally while exploring a dangerous place.
One possible scenario could be that Bella meets a new vampire who has a very different charm from Edward. Maybe this new vampire is more carefree and shows Bella a different side of the vampire world that she becomes attracted to, leading her to cheat in the fanfiction.
For Bella, a nightmare could involve her not being able to control her vampire instincts and accidentally harming someone she loves, like her father or one of her human friends. This would be a huge source of distress for her. Carlisle might have a nightmare where the Cullen family is ostracized by the vampire community for some reason, like breaking an important vampire law. He would be devastated as he has worked so hard to build a harmonious existence for his family within the vampire world.
In a 'Jacob cheats on Bella with Leah' fanfiction, the plot might start with Jacob feeling neglected by Bella who is still somewhat hung up on Edward. Leah, on the other hand, is there for Jacob, and they gradually get closer. This closeness turns into an affair, and when Bella discovers it, there could be a big fight. Bella might be heartbroken and seek revenge, or she could try to win Jacob back. Meanwhile, Jacob has to deal with his guilt and the consequences within the pack as his actions could affect pack dynamics.
In some fanfictions, Jacob might hate Bella because of the love triangle situation. He could feel that Bella has led him on or toyed with his emotions. For example, if she constantly goes back and forth between him and Edward, it can make Jacob bitter and resentful towards her.