One idea could be that your OC has a unique power set, like the ability to manipulate time in small pockets. This would create an interesting dynamic against Wonder Woman's strength and speed. For example, the OC could slow down time around Wonder Woman to gain an advantage in a fight, but then Wonder Woman could use her combat skills to counter despite the time distortion.
Maybe your OC is from a different dimension where the laws of physics are very different. In the fanfic, this OC could introduce concepts that are completely alien to Wonder Woman. They could have a battle of wits as much as a physical battle, with the OC trying to convince Wonder Woman that their way of doing things is better, while Wonder Woman stands firm in her beliefs and values.
One idea could be to create an OC who is a young Amazon in training. She could struggle with mastering the traditional Amazonian skills and look up to Wonder Woman as a role model. This OC could have a unique weapon or power that sets her apart.
One idea could be a misunderstanding. Maybe some false information makes Batman think Wonder Woman has been compromised and he has to stop her. Another could be a competition for a powerful, mysterious artifact that they both believe they should safeguard. And a third idea could be a mind - control situation where Wonder Woman is being controlled by an enemy and Batman has to fight her to break the control.
One idea could be a crossover with Superman. Imagine Wonder Woman and Superman teaming up to fight a powerful alien threat that neither could handle alone. Their different powers and personalities would create an interesting dynamic.
One idea could be that Naruto is transported to the DC universe and teams up with Wonder Woman. They could combine their unique combat skills, like Naruto's shadow clones and Wonder Woman's lasso and super strength, to fight a common enemy. Maybe a powerful villain from Naruto's world has also found a way into the DC world.
A fanfic could explore a scenario where Batman is framed for a crime on Themyscira. Wonder Woman doesn't believe he's guilty and they have to work together to clear his name while also uncovering a conspiracy within her own homeland that threatens both their worlds.
Maybe in the fanfic, there is a threat to the entire world that only the combination of Wonder Woman's Amazonian knowledge and Batman's technological resources can solve. For example, an ancient artifact has been stolen and is being misused. They start off not trusting each other fully but as they work together, they develop a deep respect.
One idea could be that Static's electrical powers interact in unexpected ways with Wonder Woman's magic lasso. Maybe when he accidentally touches it, it creates a power surge that sends them both to an alternate dimension where they have to work together to get back.
One idea could be that Hulk and Wonder Woman team up to fight a common enemy, like a powerful alien invasion. Their different powers would make for some exciting battle scenes. Hulk's brute strength combined with Wonder Woman's combat skills and magical weapons would be a great combo.
A great plot for this fanfic could be that a mysterious force is causing chaos in both Gotham and Themyscira. Batman and Wonder Woman join forces to investigate. Along the way, they discover that the source of the problem is related to an old grudge between two long - forgotten groups. As they dig deeper, they also explore their own feelings for each other and how their different approaches to justice can be reconciled.
Perhaps Batman discovers a strange artifact that has ties to Wonder Woman's homeland. He seeks her out for help in understanding it, and together they uncover a plot by an ancient evil force that was once sealed away but is now trying to return. Their journey involves not only fighting off minions of this evil but also learning about each other's worlds more deeply.