One idea could be exploring Lust's backstory before she became a Homunculus. Maybe she had a human life before and what led to her transformation. Another could be a fanfiction where Lust forms an unlikely alliance with a human alchemist. They could work together to achieve a common goal, like saving a town from a greater evil.
Power dynamics also play a role. Mustang is a high - ranking officer and Hawkeye is his subordinate. Fanfictions sometimes play with this dynamic, showing how it affects their relationship, whether it's in a military - related plot or in a more personal, romantic context. There are also stories that explore how they overcome the barriers created by their different ranks to be together.
Their relationship could develop through mutual respect. If they are constantly thrown into situations where they have to rely on each other's skills. For example, in a battle against a powerful alchemist, Lust's combat abilities and Edward's alchemical knowledge could complement each other, leading to a grudging respect that might grow into a friendship of sorts.
The OC could share a similar past with Roy. For example, if they both had a traumatic experience related to the Ishvalan War. This common ground would create an instant bond as they understand each other's pain and guilt. They could then support each other in dealing with their past traumas while falling in love in the process.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of Fullmetal Alchemist fanfictions, including those about Mustang and Hawkeye. Just use the search function and type in their names.
The OC could be an undercover agent sent to spy on Roy Mustang. However, as she gets to know him better, she realizes that he's not the power - hungry military man she was led to believe. Roy, on the other hand, discovers her true identity but instead of turning her in, he tries to convince her to join his side. This back - and - forth dynamic can lead to a very engaging fanfiction with a romantic subplot.
Her character could be developed by exploring her moral compass. Since she is a Homunculus, she doesn't have the same moral framework as a human. But what if she starts to question her actions? Maybe she meets a young alchemist who makes her think about the consequences of her deeds.
One interesting plot idea could be that Edward and Lust are forced to team up against a common enemy. Maybe a new alchemic experiment gone wrong has created a powerful and dangerous being that threatens the world they know. Their combined knowledge of alchemy and Lust's unique abilities might make for an interesting dynamic as they struggle to defeat this foe.