The conflict. The struggle between duty and desire is a very relatable concept. In the Twilight world, it adds an extra layer of depth to the characters. For example, a vampire might have a duty to their coven but a desire for a human. It creates interesting storylines.
There is a fanfic named 'Duty's Call and Desire's Temptation'. In this story, it shows how the characters' sense of duty is constantly at odds with their desires. It's a great read as it keeps you on the edge of your seat, wondering whether they will choose their duty or give in to their desires. It also has some great character development.
I haven't read this specific fanfic, but generally, a 'Twilight' fanfic might explore different aspects of the original 'Twilight' world. It could delve deeper into the primal instincts of the characters, like vampires' hunting instincts or werewolves' territorial instincts. And the 'darkest desire' part might involve some hidden, perhaps more morally ambiguous wishes of the characters.
It's likely that the main characters would include some of the original Twilight characters like Edward Cullen. Since it's a fanfic about 'darkest desire', Edward's struggle with his own vampiric desires might be explored more deeply. Maybe his desire to fully embrace his vampire nature while also being in love with Bella could be a central aspect.
There might be a character who is a bit of an outcast but has special abilities that become crucial in the story. They could be initially underestimated by others in the 'beyond duty' setting but end up being a key player.
One interesting plot could be a new vampire character with a 'darkest desire' that challenges the existing power dynamics in the Twilight universe. For example, a vampire who desires to expose the vampire world to humans for some self - serving reason.
Another theme might be self - discovery. The characters' darkest desires could lead them to discover new aspects of themselves. For instance, a werewolf might find out through his darkest desire that he has a stronger connection to the spiritual side of his nature than he thought. And the primal instinct could be a driving force in this self - discovery journey, as they learn to accept or control these instincts.
One idea could be exploring uncharted territories in the 'beyond duty' world. Maybe the characters face new and strange creatures that test their limits.