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.
A plot could revolve around Demetri being sent on a mission by the Volturi that somehow involves Leah. Maybe the Volturi suspect the werewolves are up to something and Demetri has to spy on them. But when he meets Leah, he realizes that the werewolves are just trying to protect their land and he has to figure out a way to report back to the Volturi without causing trouble for the werewolves.
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 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 exploring how Leah's pregnancy affects her relationships within the werewolf pack. Maybe there are some who are overprotective, while others are worried about how it might change the pack dynamics.
Maybe the male who imprints on Leah has some special power. His imprinting on her unlocks a latent power within Leah herself. They then have to go on a journey together to learn how to control and use this new power. Along the way, they face challenges from vampires who fear the new power they possess and werewolves who are jealous of the strong bond they share.
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.
At first, their relationship might be full of tension. Demetri is a vampire and Leah is a werewolf. But as they interact more, they could find common ground in their loyalty to their respective groups. They might start to communicate secretly to avoid causing chaos between vampires and werewolves.
One great place to find such fanfiction is on Archive of Our Own (AO3). There are many stories there that might feature Leah and Demetri sharing a kiss. You can search using their names and the keyword 'kiss' to narrow down the results. Another option could be Fanfiction.net, but AO3 generally has a wider range of works with different themes and interpretations.
Another plot could be centered around their relationship growing stronger during the pregnancy. They could discover new things about each other as they prepare for the baby. For example, Jacob might find out that Leah has a hidden talent for painting, which she uses to decorate the nursery.