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.
A possible plot could be that a powerful villain has created a device that can nullify Superman's powers under a red sun - like situation and also dampen Wonder Woman's strength. Trapped in a weakened state, they must rely on their wits and the help of some of their less - well - known allies. They then work together to reverse the effects of the device and take down the villain. This would show their vulnerability and how they can overcome it together.
Maybe there's a magical artifact that is causing chaos in the world. Superman, with his physical might, and Wonder Woman, with her knowledge of magic, join forces. They travel to different realms to discover the origin of the artifact and how to destroy it. Along the way, they learn more about each other's cultures and powers, which helps them grow as characters.
A good plot could be that the world is suddenly hit by a wave of anti - superhero sentiment. Wonder Woman and Superman are two of the main targets. They have to go undercover and live among the humans for a while. During this time, they learn more about the regular people's fears and concerns. They also start to develop a romantic relationship as they rely on each other for support in this difficult situation.
One plot idea could be a team - up where they face a new, extremely powerful villain that neither of them can defeat alone. This villain could have powers that are a combination of their weaknesses, like a being that can manipulate kryptonite and also has the strategic mind of Batman's enemies. They would have to learn to work together in a new way.
Maybe in the fanfiction, Superman and Wonder Woman get married in a secret ceremony. Then they have to deal with the reactions of their superhero friends and the public. Some are happy, while others are worried about the implications of their union on the superhero community.
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.
A crossover could start with Wonder Woman being summoned to Asgard by some mysterious magic. She finds herself in a strange new world and is initially met with suspicion. Thor, being the more open - minded of the Asgardians, decides to help her figure out how she got there. Along the way, they discover that an ancient enemy from Thor's past has plans to use Wonder Woman's power to open a portal to a realm full of chaos. They then have to work together to stop this from happening, with Wonder Woman teaching Thor some new combat techniques she learned on Themyscira and Thor showing her the power of the Asgardian magic.
A cool scenario would be a battle royale - style crossover where Wonder Woman is brought into the Marvel universe to participate in a tournament. She has to fight against various Marvel heroes and villains, like Thor and Loki. As she progresses, she discovers that the real mastermind behind the tournament is trying to use the chaos to open a portal to another dimension. Along the way, she forms alliances with some of the Marvel characters and together they try to stop the evil plan.
One plot idea could be that an otherworldly threat emerges that only their combined powers can stop. They have to put aside their differences and work together seamlessly. For example, a powerful alien race plans to invade Earth and they need to use Batman's strategic mind, Superman's super strength and Wonder Woman's combat skills.
One idea could be that an alternate - universe version of Superman comes to their world. This could lead to a power struggle as both Supermen have different ideas on how to protect the city. Another plot could involve Lois getting superpowers temporarily and having to learn how to use them while also dealing with her regular reporting job.
Here's an idea. There could be a powerful magical artifact that surfaces in both cities. It has the power to change reality. Superman, being a man of science, is initially skeptical about its power. Batman, on the other hand, has seen enough strange things in Gotham to know it's dangerous. They have to overcome their differences in beliefs as they race against time to stop different factions in both cities who want to use the artifact for their own gain. Along the way, they also learn more about each other's worlds and form a stronger bond.
One idea could be that Superman and Invincible have to team up to stop an alien invasion that threatens both their worlds. The two heroes could initially clash due to their different methods but then learn from each other.