The Cheetah could also be a main antagonist. She could be working with some anti - hero or anti - superhuman group. She uses her speed and cunning to set up situations where Batman and Wonder Woman seem to be causing trouble. For example, she could steal something and make it look like they did it.
Darkseid is another option. He could use his mind - control powers on some of the public and world leaders. He makes them believe that Batman and Wonder Woman are planning an invasion or something evil. Batman and Wonder Woman then have to fight against Darkseid while also trying to convince the world that they are on their side.
Lex Luthor could be a great main antagonist. He could be jealous of the trio's popularity and see them as a threat to his own power - hungry plans. He might create elaborate traps and use his vast resources and intellect to try and take them down, all while trying to make it look like they are the bad guys in the public eye.
There could be an ancient evil that is awakened by the birth of their son. This entity could see the son as a threat to its plans for world domination. It might have powers that are beyond what the son initially thinks he can handle, but through his own growth and the help of his parents' friends, he is able to overcome this powerful antagonist.
The Joker could also be a main antagonist in this fanfic. He could be interested in causing chaos not only in Gotham but also in the world of Wonder Woman. His unpredictable nature and love for mayhem would make him a formidable foe for the two heroes. He might come up with a plan that plays on their differences and tries to drive a wedge between them while also causing destruction.
It could be Lex Luthor. He's always looking for ways to take down superheroes. He might manipulate Wonder Woman into believing Batman is a threat, so she betrays him. Luthor is smart enough to create a complex scheme to drive a wedge between them.
One idea could be that a new supervillain frames Batman and Wonder Woman for crimes against the world. They have to work together while on the run to clear their names. The villain could be using mind - control technology on world leaders to make them believe Batman and Wonder Woman are the threats.
Their relationship could be shown as a very professional partnership. They respect each other's skills and intelligence. Batman admires Wonder Woman's strength and her connection to the mythological world, while Wonder Woman respects Batman's detective skills and his determination to protect Gotham.
Maybe in the fanfiction, there's a situation where Batman discovers a secret about Wonder Woman's past that could put her in danger from their common enemies. They then have to work together to keep that secret safe while also dealing with the threats from the 'Public Enemies' scenario. For example, they might have to face off against Lex Luthor who wants to use this secret to his advantage.
Perhaps it could be an ancient god who has been awakened from a long slumber and wants to take over the world. This god would have powers that rival Wonder Woman's.
Lex Luthor could be a great antagonist. He could be jealous of the new alliance between Wonder Woman and Batman and see them as a threat to his own plans for world domination. He might use his intellect and resources to try and turn the public against them or create some high - tech weapons to take them on.
The Cheetah could be a good choice too. She could be working with some high - tech criminals to steal some powerful artifacts. Her speed and ferocity combined with the new allies she has made could put Batman and Wonder Woman in a tough spot as they try to stop her from achieving her evil goals.