Ra's al Ghul could also be a main villain. He might see the child of Batman and Catwoman as a potential weapon or a threat to his plans for Gotham. He could try to either recruit the child to his side or eliminate them, which would put Batman and Catwoman in a difficult position as they fight to protect their family from Ra's al Ghul's devious schemes.
One possible plot could be that Batman and Catwoman have a child who discovers their parents' secret identities by accident. The child might then try to use their own skills, a combination of Batman's detective skills and Catwoman's agility, to help their parents fight a new villain in Gotham.
The Penguin could be a main antagonist. He might see the daughter as a new threat to his criminal operations in Gotham. Since she has the combined skills of Batman and Catwoman, she could easily disrupt his illegal business, like his smuggling operations.
The Joker could be a main villain. He might see Izuku as a new plaything and try to break his spirit during his training with Batman.
Perhaps an old enemy of Batman who has made a deal with Cadmus. They combine their knowledge and resources. This could be someone like Mr. Freeze, who has found a way to use Cadmus' genetic engineering to make himself even more powerful. His new form and abilities would pose a great challenge for Batman Beyond.
Lex Luthor would be a great choice. He's Superman's long - time nemesis. He could form an alliance with some of Batman's foes like the Joker. Luthor could use his intelligence and resources to create a plan that only Superman and Batman together could stop.
The Reverse - Flash could be a great main villain. He could travel to Gotham to cause chaos and draw out the Flash. His time - manipulation abilities would pose a huge threat not only to the Flash but also to Batman's carefully planned strategies in Gotham. Batman would have to adapt to dealing with a villain who can move at super - speed and mess with time.
The Joker could be a great villain. Imagine if the Joker - like character emerged in the My Hero Academia world. Izuku, with his new Batman - inspired skills, would have to use his wits to outsmart this chaotic and unpredictable foe.
Ra's al Ghul could also serve as the main antagonist. He might have a new plan for Gotham that involves using Catwoman as a pawn. He could be aware of her relationship with Batman and try to use it to his advantage. Batman would have to not only save Gotham but also rescue Catwoman from Ra's al Ghul's clutches.
Mainly, it's Catwoman and Batman. Catwoman, Selina Kyle in her civilian identity, is a character who straddles the line between good and bad. She has her own code of ethics. Batman, Bruce Wayne by day, is the symbol of justice in Gotham. In this type of fanfiction, their relationship is at the heart of the story. Their different outlooks on life, Catwoman's more carefree and self - interested approach compared to Batman's rigid sense of justice, create interesting dynamics and conflicts as they come together in the story.
Obviously, the main characters are Batman and Catwoman. Batman, the Dark Knight of Gotham, known for his brooding nature and fight against crime. And Catwoman, the agile and mysterious thief who often walks the line between good and bad.