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.
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.
A dominant Jacob is usually in touch with his alpha side. This means he has a natural leadership quality. He can command respect from others in the fanfiction world. And when it comes to Bella, this leadership translates into him guiding their relationship in a way that he believes is best for both of them. He may be more confident and self - assured, which can be both appealing and a source of conflict in the relationship with Bella.
Sure. There might be some new characters created specifically for this fanfiction. Maybe a wise elder in the werewolf community who gives advice to Jacob about how to handle the situation with Bella and the pregnancy. Or perhaps a human friend of Bella's who discovers the truth and has to deal with the shock of the supernatural world.
Bella herself might struggle with her identity. She has been so caught up in the vampire world before, and now being with Jacob, a werewolf, she has to redefine who she is and where she belongs. She may also worry about how her human family and friends will react to her being with a werewolf.
One character could be Seth Clearwater. As a member of Jacob's pack, he could play an important role in their married life. Maybe he helps them out in times of trouble or is like a younger brother figure to their family.
It would become more intense and complex. Jacob would be very protective, his werewolf instincts kicking in fully. He'd be constantly worried about Bella and the baby's safety.
It could be that the fanfiction writer took a different approach to the story. In some fanfictions, the Cullens could be involved in situations that go against what Bella stands for. Perhaps they are written to be more secretive and untrustworthy, leading to Bella's hatred.