One popular fanfiction could be about Bella's struggle between her vampire nature and her remaining feelings for Jacob. For example, a story where she tries to protect Jacob from the vampire world while still being drawn to him in a way that defies her new vampiric instincts.
There might be a fanfiction that focuses on Jacob's reaction to Bella fully becoming a vampire. Maybe he tries to find a way to reverse it, or he has to come to terms with loving her in this new form. In this kind of story, there could be a lot of emotional turmoil as Jacob battles his own wolf instincts and his love for Bella.
There is a fanfiction titled 'Bella's New Path with Jacob's Imprint'. This one delves deeper into how Bella copes with Jacob's imprint. It shows her internal struggle between her previous feelings for Edward and the new, intense bond with Jacob. It's a more emotional and character - driven story.
Another great one is 'The Scarred Heart'. Here, Bella's words during an argument deeply wound Jacob. The story then follows Jacob as he tries to heal while still being in Bella's orbit. It's a poignant exploration of their complex relationship.
One popular 'Bella Swan and Jacob Black mates' fanfiction could be 'The Alternate Bond'. It explores a different path for their relationship, where they overcome various obstacles together. Another might be 'Jacob's True Mate' which delves deep into the idea of Jacob's unwavering love for Bella in a more unique setting. There's also 'Bella's Choice for Jacob' that shows Bella making a conscious decision to be with Jacob and how that changes the whole Twilight - like world they live in.
In the fanfiction 'Jacob's Unwavering Imprint', the focus could be on how Jacob's imprint on Bella is unshakable despite the many difficulties. It might show how Jacob is willing to go against the norms of his pack and even face danger to protect Bella. This fanfiction could also explore how Bella starts to rely on Jacob in ways she never expected due to the imprint.
One plot idea could be Bella struggling to choose between her vampire life with Edward and her deep friendship with Jacob that has the potential to turn into more. Another could be Jacob trying to protect Bella from a new vampire threat while also dealing with his feelings for her. Maybe a plot where Bella discovers a new power as a vampire that Jacob has to come to terms with.
There are fanfictions where Jacob and Bella actually form a strong friendship early on, and they work together to protect their tribe and the Cullens from a new, unknown threat. This could involve them training together and sharing their unique skills.
Some fanfictions in this theme might focus on the reactions of the Cullen family. They could be either accepting or having some sort of conflict due to Bella's situation with Jacob. Also, the story could explore how Bella herself copes with the physical and emotional changes of being pregnant, especially in a world filled with vampires and werewolves.
I've read one where Bella discovers a new power as a vampire that allows her to communicate with Jacob telepathically. This completely changes the dynamic between them as they can now share their innermost thoughts and feelings without having to be in close proximity. It also makes their relationship more complex as they have to deal with this new form of connection.
One great 'Bella Swan and Jacob Black Fanfiction' is 'The Wolf's Mate'. It delves deeper into their relationship outside of the main story, exploring Jacob's perspective more. Another good one could be 'Jacob's Redemption'. It shows how Jacob tries to win Bella back in a different way. And 'Bella's Choice' is also interesting as it presents a scenario where Bella has to make a tough decision between Jacob and her other life choices.
Bella's actions in the fanfiction might be another cause. Maybe she accidentally says or does something that undermines Jacob's self - esteem. Like if she makes a comment about his werewolf nature in a negative way, it could wound him deeply as it's an important part of who he is.