A crossover with Batman could also be great. Wonder Woman could bring her Amazonian wisdom and combat skills to Gotham City. They could work together to take on a villain like the Joker who has a new, more global and dangerous plan. The contrast between the dark and brooding Batman and the heroic and idealistic Wonder Woman would be fascinating to explore.
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 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.
Another possibility is that there is a villain who is stealing technology from Stark Industries (where Spiderman sometimes gets his tech) and ancient artifacts from Wonder Woman's homeland. They are both independently investigating and then cross paths and decide to work together to stop the thief.
A good crossover story idea could involve a cosmic entity that is causing chaos in both the DC and Marvel universes. Wonder Woman, with her Amazonian strength and wisdom, teams up with the Avengers, each bringing their unique skills to the table. For example, Captain America's leadership and combat skills combined with Wonder Woman's combat prowess would make for an interesting dynamic. Or perhaps a villain has found a way to merge the two universes in a dangerous way, and the only way to stop it is for the Avengers and Wonder Woman to find a way to separate them again while also dealing with the new threats that have emerged from the merge.
One idea could be that Superman and Wonder Woman have to team up to stop an ancient alien threat that is immune to their individual powers. They must combine their strength, speed, and wisdom to find a way to defeat it.
One idea could be that a powerful new villain emerges who is a threat to both Gotham and Themyscira. Batman and Wonder Woman team up to track down the villain's lair. Batman uses his detective skills to analyze the clues while Wonder Woman uses her combat skills in the battles along the way.
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.
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.
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.
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.