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.
A plot could involve Jane and Demetri getting lost in a strange part of the world. They have to rely on each other to survive and find their way back to the Volturi. During this time, they discover new powers within themselves and also develop a deeper bond that was previously just based on their loyalty to the Volturi.
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 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.
A plot idea could be that Jane and Alec get separated from the Volturi during a mission. They then have to find their way back while facing various challenges, like other vampire clans or even some human hunters who are aware of their existence. This would force them to rely on each other more than ever.
The characters themselves are very interesting. Demetri with his tracking abilities and Jane with her unique power of causing pain through illusion. Fanfiction allows writers to explore their backstories and relationships in more depth than the original books or movies. For example, a fanfic might delve into Demetri's life before he became a vampire.
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.
Well, there are also some dedicated Twilight fanfiction forums out there. These forums often have sections specifically for fanfictions about different characters. You might have to register on some of them, but it's worth it. Once you're in, look for threads related to Demetri and Jane. Members often discuss and share their favorite fanfictions in these threads, which can lead you to some great reads that you might not find on the more mainstream platforms.
Another way is to put them in situations that challenge their loyalty to the Volturi. In the Twilight world, the Volturi are very powerful and feared. But if Jane and Demetri are faced with a situation where they have to choose between doing what the Volturi want and what they believe is right, it would show different aspects of their characters. They could also interact more with other characters outside of the Volturi, which would force them to adapt and grow.
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.