One popular story might be one where Jacob and Bella face many challenges together while expecting a baby. Their relationship could be explored in-depth, like how they deal with the reactions from their families and friends in the fanfiction.
You can try some popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions and there's a good chance you'll find some related to this topic there.
I'm not aware of it being a particularly popular fanfiction. There are so many 'Twilight' fanfictions out there, and ones that deviate so far from the original story's relationships may not gain mainstream popularity.
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.
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.
I'm not sure specifically as it's a fanfiction. Fanfictions can have a wide variety of interpretations and storylines. It might explore an alternate scenario where Charlie Swan and Bella are in a relationship that leads to such a situation, but this goes against the original 'Twilight' story's intended relationships.
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.