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.
In the fanfiction, perhaps Wonder Woman is trapped in some sort of magical or technological prison created by an enemy. Batman, with his detective skills, figures out how to break in and free her. He might have to solve a series of puzzles or overcome various security measures. He then rescues her and they team up to take on the bad guy who imprisoned her.
Lex Luthor could be one of the main villains. His intelligence and thirst for power often put him at odds with both Wonder Woman and Batman.
Lex Luthor could be a likely villain. He's always devising plans to take down the Justice League, and this time he might have come up with a scheme that almost succeeds, until Batman steps in. For instance, he could create a super - weapon that targets the unique powers of each Justice League member.
One possible plotline could be that Batman sacrifices himself to save Wonder Woman from a powerful enemy. Maybe he takes on a self - destructing device or holds off a horde of villains while she escapes. Another could be that he is poisoned by a new and deadly toxin, and in his final moments, he imparts some crucial knowledge or advice to Wonder Woman.
A possible storyline could be that Wonder Woman comes to Gotham to investigate some strange occurrences that seem related to her world. She teams up with Batman, and they discover a cult that is trying to summon an ancient evil. Along the way, they learn to respect each other's methods.
One possible villain could be Orochimaru. He's always been interested in powerful ninja for his own experiments. Nawaki might have some special potential that Orochimaru wants to exploit, and Naruto has to stop him.
Their relationship could become more trusting. Since Batman saves Wonder Woman, she might start to see him as more than just a brooding vigilante. She could open up more to him and share her fears and secrets.
One possible storyline could be that they are facing an extremely powerful and ancient evil. This evil has the power to corrupt the very essence of the world. Batman and Wonder Woman, in their attempt to stop it, find themselves in a situation where they have to sacrifice their lives. Maybe they are lured into a trap where the only way to prevent the evil from spreading across the universe is for them to use their combined powers in a self - destructive way.
Maybe it takes place in an alternate universe where the world is a bit different. Batman is more open with his emotions. He takes Wonder Woman to a special place, like an ancient, hidden garden. There, he tells her how much she means to him, not just as a teammate but as someone he has romantic feelings for. He might talk about how her strength and beauty have always attracted him, and how he has been trying to hide these feelings for a long time but can't anymore.