Darkseid is another possibility. He's a powerful and menacing villain. If he sets his sights on Wonder Woman, Batman would be forced to act. He'd need to use all of his skills and resources to save her from Darkseid's Omega Beams and his army of minions.
Cheetah would be a great villain to include. Her connection to Wonder Woman's world and her fierce fighting skills would make for some exciting battles. Also, Bane could be interesting. His brute strength and strategic mind could pose a real challenge to Batman and Wonder Woman when they team up. Lex Luthor is another option. His intelligence and vast resources could create a complex and dangerous situation for the two heroes.
Bane. He was born and raised in a prison called Peña Duro. He was forced to endure harsh conditions from a young age. To survive, he developed great physical strength and a cunning mind. He has a vendetta against Batman. His origin story fuels his desire to break Batman both physically and mentally. He is often seen as one of the most physically imposing of Batman's foes, and his backstory gives him a unique motivation for his actions.
Catwoman's origin has different interpretations. In some versions, she grew up in poverty and turned to thievery as a means of survival. Her skills as a cat burglar are what initially put her at odds with Batman. Over time, she has also developed a complex relationship with Batman, sometimes being an ally and other times a foe. Her feline - inspired persona and acrobatic skills make her a unique and iconic Batman villain.
The Joker is a very well - known main villain. He's completely unpredictable and his chaotic nature provides a great contrast to Batman's order - seeking personality.
Catwoman, Selina Kyle. Her origin is rooted in poverty. Growing up in a tough part of Gotham, she had to steal to survive. She became an expert thief. Her relationship with Batman is complex as she is a criminal but also has a sense of justice. She often straddles the line between good and evil, stealing valuable items but also sometimes helping Batman in her own way.
Orochimaru could be like the Joker. He's already a bit of a mad scientist and likes to play with people's lives. Just as the Joker likes to create chaos, Orochimaru could do the same in Konoha, and Naruto, as the Batman - inspired hero, would have to stop him. He could also have some henchmen like the Joker does.
Another great one is 'A Death in the Family'. This story focuses on Jason Todd, the second Robin, and his encounter with the Joker. The Joker brutally kills Jason Todd, which has a huge impact on Batman. It's a story full of emotion, from Batman's grief to the shock of the readers. It also explores the darker side of Batman's world and how far he's willing to go for justice.
One of the most frequently used villains in Batman fanfiction is Two - Face. His split personality makes for great internal conflict within the character. Scarecrow is also popular as his use of fear toxin can create really intense and spooky storylines. Killer Croc, with his monstrous appearance and his outcast status, can also be an interesting villain to write about in fanfiction.