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.
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.
Their relationship might start off a bit rocky. Batman is used to working alone and Billy is a free - spirited kid. But over time, Batman's strict discipline could help Billy focus his powers better. Billy, on the other hand, could show Batman that there's more to life than just fighting crime in the shadows. They'd gradually develop a deep bond of trust and respect for each other.
I'm not sure if there is a specific, well - known 'Billy Batson is Batman's son' fanfiction, but in the world of fanfiction, anything is possible. Some fanfictions might be part of a larger crossover universe where different superhero properties are mashed up. This particular concept could lead to some really interesting storylines, like how Billy would deal with Batman's strict code, or how Batman would handle having a super - powered son who also has a very different personality compared to his usual sidekicks.
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.
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.
The theme of self - doubt can be seen in Billy Batson hurt fanfiction as well. After experiencing a difficult situation, he may start to doubt his abilities as Shazam. He could question whether he is truly worthy of the power, which is a central aspect of his character, and this self - doubt can cause him internal turmoil and more 'hurt' in an emotional sense.
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.