One idea could be that as Billy turns into a female, she has to deal with different social expectations in Fawcett City. For example, she might find that people treat her differently when she's out as Shazam in her new form.
A possible plot could involve Billy, now female, struggling with her identity. She was used to being a boy and now has to come to terms with being a girl. This could be explored through her relationships with her friends at school like Freddy. She might worry about how he'll react and if their friendship will change. Also, she could have a whole new perspective on the villains she faces as a female hero, perhaps seeing different aspects of their misdeeds that she didn't notice before.
One idea could be Billy Batson facing a new villain that the Justice League has trouble dealing with on their own. Billy's unique powers as Shazam could be the key to victory. For example, the villain might have the ability to nullify the powers of the Justice League members one by one, but Billy's magic, being different in nature, remains unaffected.
Well, a really interesting plot might be Billy Batson losing his ability to transform into Captain Marvel. He then has to find a way to regain his powers, perhaps by going on a quest that involves traveling to ancient magical places. Along the way, he could learn more about the origin of his powers and the history of the wizard who gave them to him. He might also encounter other young heroes from the Young Justice team who are initially skeptical of his ability to be useful without his powers but then come to see his true worth as Billy, not just as Captain Marvel.
One possible plot could be that as a female, Billy (let's call her Billie now) discovers new aspects of the Shazam powers. Maybe she has a different approach to using the powers for good in Fawcett City. She might face unique challenges from the villains who are not used to a female Shazam. For example, they might underestimate her at first, but she quickly proves them wrong with her newfound strength and the wisdom of Solomon which she uses in a more empathetic way as a female.
One idea could be that Billy Batson, as Shazam, has to team up with Constantine to fight a demon that is causing chaos in the city. The demon is using magic that is affecting the ley lines, and only by combining Shazam's strength and Constantine's magical knowledge can they stop it.
Dr. Sivana could still be the main antagonist. His hatred for Shazam wouldn't change just because Billy is now female. In fact, he might be even more enraged or intrigued by this new development and try to take advantage of it in some way.
Another storyline might involve Batman and Billy Batson teaming up to take on a common enemy that neither could defeat alone. Maybe a powerful villain who has the ability to manipulate magic and technology at the same time. Their combined skills, Batman's detective work and Billy's superhuman strength, would make for an exciting adventure.
A plot twist could be that the family that adopts Billy has a secret connection to the wizard who gave him his powers. Maybe they are descendants of the wizard's helpers, and this connection is slowly revealed as Billy grows up in the family.
One idea could be that as a female, Luffy has to deal with different expectations in the pirate world. For example, some male pirates might underestimate her at first but then be amazed by her strength. Another could be her relationships with the crew change in new ways. Maybe Sanji is even more protective or Zoro has a different kind of respect for her. Also, her appearance might cause different reactions from other pirates or even the World Government, leading to new challenges and adventures.
One popular theme is Billy's relationship with his foster family. It often explores how they support him in his dual life as a normal kid and Shazam. Another theme is his adventures with other DC heroes, like teaming up with Superman or Batman to fight a common enemy.
The relationship with her foster family might change. They could be more protective of her as a female, or she might have to prove herself in a different way. Her interactions with the villains would also be different. They might target her differently, like using more psychological attacks based on her gender. For example, they might try to make her doubt her abilities as a female superhero.