I'm not sure specifically what 'resumen secret story' refers to without more context. It could be about a secret story in general, perhaps a summary of a mystery or a hidden tale.
One common element is a spooky location like an old house, a cemetery or an abandoned building. These places are often associated with the unknown and death.
A great part of Homunculus Edward fanfiction is the creativity in the plot twists. Writers can introduce unexpected elements. Like, what if Edward's alchemy was affected by a homunculus in a way that made him able to do things he never thought possible? This can lead to really engaging stories that keep readers on their toes.
The hitmen Vincent and Jules are key. Their relationship and their jobs form a big part of the story. Also, the briefcase they carry around is important as it holds something mysterious. Another key point is Butch the boxer and his decision not to throw the fight as he was supposed to.
To create a compelling OC homunculus, start with its motivation. What does it want? Is it power, acceptance, or something else? Then, think about its relationships. Does it have an ally among the human alchemists or is it at odds with all of them? Also, consider its powers. Make them interesting and related to the Fullmetal Alchemist lore. For instance, it could have the ability to manipulate a specific type of metal in a way that's different from the main characters. And don't forget to give it flaws. A homunculus that is too perfect won't be as engaging.