One idea could be that Ed has to keep her true gender a secret while on her alchemic adventures to avoid discrimination in a male - dominated alchemic world. Another could be that she forms unique relationships with other characters who might suspect or eventually discover her secret.
Alphonse could be a major supporting character. Since he is Ed's brother, he would be in a unique position to either help her keep the secret or accidentally almost expose it. Winry could also be important. She might be someone Ed could confide in or struggle to keep the secret from, especially if their relationship takes on new dimensions due to Ed's secret gender.
The plot could also involve Ed using her new - found female identity to her advantage in some situations. For example, in a world where people might underestimate a girl in alchemy, she could surprise them with her skills. Or she could use her femininity to gain access to certain places or information that were previously unavailable to her as a boy. This could create all sorts of interesting twists and turns in the story as she navigates the world of alchemy with this secret identity.
One idea could be that as a girl secretly, Ed Elric might face different challenges in the male - dominated alchemist world. Maybe she has to hide her true identity while trying to achieve her alchemical goals.
One possible plotline could be that Ed has been hiding her true gender to fit into a male - dominated society or group. Maybe she's afraid of being discriminated against. Another could be that she falls in love with someone who initially only sees her as a male friend, and then there's the big reveal.
Perhaps a jealous rival. This person might be a male who is threatened by Ed's success or popularity. When they discover Ed is a girl, they could use this secret to try and drive her out of the group or community she's in. They might spread rumors or create problems for her.
There are many fanfictions where Ed is portrayed as a girl. Fanfiction writers like to play with the existing characters in new ways. By making Ed a girl, they can introduce new themes and perspectives. For example, it could change how the story deals with gender roles in the world of Fullmetal Alchemist. Also, it can create new emotional arcs for the character, as a female Ed might face different challenges and experiences compared to the male version in the original story.
In this fanfiction, her role as an alchemist might be perceived differently. As a girl, she might face more discrimination in the male - dominated field of alchemy. This could lead to stories of her having to prove herself more often and in different ways than in the original story.
Well, it depends on what you consider 'good'. Some people might enjoy the darker, more complex take on the characters in these types of fanfics. However, not all fanfic writers are equally skilled. You may need to sift through a few to find one that really grabs you. For example, some might focus more on the psychological aspects of Ed becoming a 'criminal' and his interactions with Envy, while others might be more action - oriented.
In a fanfiction like this, Ed dressing as a girl could be part of a bet. His friends might have dared him to do it, and at first he's all about proving them wrong and showing he can pull it off. But then he realizes that wearing a girl's dress comes with its own set of challenges, like walking in different shoes or dealing with long hair (if he has a wig). This could make for an interesting character - development story.