Well, perhaps the fanfiction plot could center around Billie Batson's origin story as a female. It could be that the wizard who gave her the power originally intended for a male hero, but due to some magical quirk, she becomes female when she says 'Shazam'. This change could lead to a different set of character developments. She might have to learn to balance her femininity with the powerful superhero she becomes. And she could also inspire other female characters in the story, showing them that they can be strong heroes too.
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.
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.
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.
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 new villain could be the kidnapper. Maybe a powerful interdimensional being who has heard of Shazam's powers and wants to study Billy to find a way to bring those powers to his own dimension. This being would be extremely powerful and Billy would have to find a way to outsmart him using his own wits and the powers of Shazam.
The theme of redemption can also be found in some billy batson fanfiction. Sometimes there are characters who have made mistakes in the past and Billy helps them find a path to redemption. Additionally, the exploration of the magical world that gives Billy his powers is a common theme. Writers often expand on the magic, its rules, and its history in their fanfictions.
Billy Batson joining Young Justice in fanfiction would be really exciting. First off, his powers are quite different from what the existing team has. His lightning - based powers and superhuman abilities would give the team an edge. He might also introduce a new set of relationships. Since he's used to being on his own or with just his foster family, he'd have to learn to work in a team environment. There could be some interesting storylines around him teaching the others about his powers and learning from their skills as well.
Well, they could team up to fight a common enemy. Maybe a powerful villain that neither Young Justice nor Billy Batson could handle alone. Billy could transform into Shazam and use his super strength, flight, and lightning powers in combination with the skills and powers of the Young Justice members like Robin's detective skills or Superboy's strength.
Another place could be FanFiction.net. There are many fans who post their creative works there. However, the quality and content can vary widely. You might have to dig a bit through different categories related to the DC universe to find the specific fanfics about these two characters.
Well, it could be a story where Billy Batson, with his powers as Shazam, looks up to Batman. Maybe he tries to learn from Batman's detective skills. He could see Batman as a role model in the world of crime - fighting, despite having very different powers. Billy might even try to team up with Batman in some undercover mission where his ability to transform into an adult superhero would come in handy.
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.