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.
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 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 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.
To develop Demetri further, you could explore his past. Maybe he had a human life before being turned that he barely remembers, but certain events in the fanfiction could trigger those memories, adding depth to his character. For Heidi, you could show more of her vulnerability. She's often seen as this seductive figure, but what if she has insecurities about her role in the Volturi?
A really interesting plot could be that Heidi stumbles upon an ancient prophecy. As she tries to understand it, she has to face challenges that test her courage and kindness. The prophecy could be related to saving the beauty of the Alps from some kind of threat.
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 getting caught in the middle of a Hogwarts mystery. Maybe he's sent on a mission by the Death Eaters that somehow intersects with Harry's search for a Horcrux. Another could be a story where Demetri has a change of heart and decides to help Harry, seeing the error of Voldemort's ways.
One interesting plot idea could be exploring how different characters adapt to the post - apocalyptic world in new and unexpected ways. For example, a character who was once a timid office worker becomes a fearless scavenger.
A plot could involve a new vampire in the Twilight universe who is ashamed of their transformation and uses a mask to try and fit into human society again. They struggle with their new instincts while trying to keep their identity a secret. Along the way, they meet other characters from the Twilight series and their story unfolds in a way that challenges the norms of the original story.
One idea could be a new vampire character being reborn in the Twilight universe with a special power, like the ability to control the weather. This would create a whole new dynamic as they interact with the Cullens and other vampires.