Well, it could be about how Bella and Jane overcome the differences between their two characters. Maybe they start off as enemies since they are from different sides in the original story, but then gradually find common ground and develop a strong bond as mates.
There could be characters from the original stories that Bella and Jane come from. For Bella, characters like Edward or Jacob might still play a role, perhaps in the form of her past relationships that now create some sort of complication in her new relationship with Jane. From Jane's side, other Volturi members could be involved, like Aro or Caius, who might have opinions on Jane's new mate.
Well, it could be about Bella having an adventure with the three kings. Maybe they are on a quest together to save their land from some dark force. The three kings might bring different skills and powers, and Bella could be the key to uniting them in their mission.
Maybe the plot could involve a world where vampires have a different set of rules about mates. Victoria discovers that Bella is her mate, and she has to protect Bella from the other vampires who don't approve of this unusual pairing. Bella, in the meantime, has to come to terms with being mated to a vampire she once feared. There could be a lot of internal struggle for both characters, and they might have to fight against the prejudices of the vampire society together.
Well, it could be about Bella having to face many challenges as the mate of the God of War. Maybe she has to learn combat skills or deal with the jealousy of other characters in that fictional world.
One common theme could be the struggle for dominance between the mates. Since it's possessive, there might be a lot of tension regarding who has more control or claim over the other.
The plot may revolve around how Bella discovers she is the alpha's mate. It might start with some strange occurrences or feelings she has. Then, as she accepts this new role, she has to learn about the werewolf culture and the responsibilities that come with being the alpha's mate. This could include helping the alpha lead the pack, dealing with internal pack politics, and perhaps even facing threats from outside the pack.
It could be a new character entirely. Maybe a hybrid of some sort, half - vampire and half - werewolf, which would be a very unique and interesting choice for her true mate. This would bring a whole new set of dynamics to the story as Bella has only been involved with either vampires or werewolves before.
Perhaps the main plot is centered on a new threat in the vampire world. Bella, as a nurse, might be the key to finding a cure or solution because of her medical knowledge. Emmett, as her mate, is by her side throughout this journey. Along the way, they face many obstacles, both from the outside world and from within their relationship. Their love grows stronger as they overcome these challenges together.
Perhaps there's a plot twist where Bella's human past comes back to haunt her in this new role. For instance, old enemies from her human days find out about her new family and try to use Jane and Alec against her. This would add a whole new layer of conflict and drama to the fanfiction.
In a 'Bella Swan sister witch twins mate' fanfiction, the plot could start with the sister witch having a premonition about the twins. She realizes that their mating is crucial for some greater purpose in the supernatural world. Bella is initially skeptical but soon gets involved. As they get closer to the twins, they find out that there are ancient prophecies surrounding them. The group has to fight against an evil coven or a group of rogue werewolves who want to prevent the twins from mating. The sister witch uses her powers to fight, Bella uses her knowledge of the Twilight world, and the twins have their own unique abilities that come into play. Throughout the story, the relationships between all the characters evolve, with love, friendship, and loyalty being tested.