Maybe in a fanfiction, Demetri discovers that Bella has a hidden power that not even the Cullens are aware of. He then has to decide whether to report it to the Volturi or keep it a secret and help her develop this power. This could create a complex relationship between them, full of trust issues and moral dilemmas.
One idea could be that Bella is forced into the wedding with Demetri due to some vampire politics. But as time goes on, she discovers a softer side to Demetri that she didn't expect. Another could be a time - travel plot where Bella from the future comes back to stop the wedding but ends up falling in love with Demetri in the past. Or perhaps there's a mix - up where Bella thinks she's marrying Edward but it turns out to be Demetri at the altar, and then they have to figure out their relationship from there.
One idea could be Demetri and Leah teaming up to protect the Forks area from a new threat that neither the vampires nor the werewolves have faced before. They might start off as reluctant allies but gradually develop respect for each other.
One idea could be a story where Demetri and Heidi are sent on a secret mission by the Volturi. They have to infiltrate a group of rogue vampires who are causing chaos in a small town. Along the way, they start to question their loyalty to the Volturi as they see the good in the town's people.
One idea could be that Jane and Demetri are sent on a secret mission by the Volturi. They have to infiltrate a group of vampires who are suspected of planning an uprising against the Volturi. Along the way, they start to question their loyalty to the Volturi as they see the other vampires' reasons for wanting change.
One idea could be that Bella and Rosalie have to team up to protect the Cullen family from a new vampire threat. They start off disliking each other but as they face challenges together, they develop a mutual respect.
There could be a story where Caius has a change of heart after meeting Bella. In the Twilight world, Caius is known for his strict and often cruel ways. But when he encounters Bella, he starts to question his own beliefs. Maybe Bella gets lost in the Volturi territory and Caius finds her. Instead of punishing her, he is intrigued by her. This could lead to a story of him trying to understand her human - vampire relationship with Edward from a different perspective. He might even start to envy Edward in a way, which could create a whole new dynamic in the fanfiction.
One plot idea could be that Felix, instead of being a villain, becomes Bella's secret protector in the Twilight world. He might sense some unique power within her that even the Cullens are unaware of. This could lead to a series of adventures where they have to hide their alliance from both the Volturi and the Cullens.
One idea could be that Bella is a vampire from the start but has been in hiding. She could be on a mission to uncover a secret vampire society that has been causing trouble in the Twilight world. This would change her relationship with the Cullens as she'd be more of an equal, not a fragile human.
One idea could be Carlisle having to protect Bella from a new vampire threat that is specifically targeting her for her unique connection to the Cullens. Maybe a group of nomadic vampires who are jealous of the Cullens' lifestyle.
In some fanfictions, Bella could be turned into a vampire by Demetri in a rather unorthodox way. This new vampire Bella then has to navigate the complex vampire society with Demetri as her guide. He teaches her how to use her new abilities and they end up having a deep bond that is more than just a typical vampire - human relationship.