Ra's al Ghul could be a great choice. He might see the daughter as a potential heir or a threat to his plans for Gotham. He could try to manipulate her or use her against Batman. This would force Batman to not only protect his daughter but also try to make her understand the true nature of Ra's al Ghul.
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.
Maybe the Riddler. He could set up a series of elaborate riddles that not only challenge Batman but also the curious kid OC. The kid's fresh perspective might actually help Batman solve the riddles faster and catch the Riddler before he executes his grand plan.
In the fanfiction, the daughter might be from an alternate timeline. Batman meets her when she accidentally travels to his Gotham. She has her own set of skills, which she learned from stories about her father. They team up to fight a new supervillain who is threatening the city, and during this time, Batman realizes how much he missed out on having a family life.
It could be someone like Ra's al Ghul. He is a powerful and complex villain that both Batman and The Shadow would have a hard time dealing with. His global influence and long - life plans would pose a great threat to their efforts to protect Gotham and the world.
The Joker is a likely suspect. He loves to target Batman's loved ones to mess with him. He might use the kidnapping as a way to draw Batman into a twisted game, setting up elaborate traps along the way.
Another option could be a new villain created specifically for the fanfic. This villain could be someone who has a vendetta against the Wayne family because of something Batman's dad did in the past. He could be a master of disguise and manipulation, making it difficult for Batman and his dad to figure out who he really is.
There could be Chucky's old enemies who might try to use his daughter against him. For example, some of the people he wronged in his past murderous sprees.
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.
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.