There could be a plot where the OC is from another dimension. This OC has special powers and lands in Gotham. Batman initially suspects the OC but as time passes, they start to trust each other. Meanwhile, characters like Poison Ivy and Harley Quinn are drawn to the OC's otherworldly charm, leading to the formation of a harem. The group then has to deal with threats that cross dimensions.
In this type of fanfiction, a plot might be that Batman's harem is formed by various female superheroes or characters associated with the Justice League. The story could explore how their different personalities and powers interact within the harem. For instance, Wonder Woman might bring her Amazonian values and leadership skills, while another character could offer a more tech - savvy or street - smart perspective. And all of this while they are in a relationship with Batman and also working together in the Justice League.
One interesting plot could be the sons teaming up to solve a mystery that Batman himself has trouble with. They might use their unique skillsets, which are different from Batman's, to approach the case from a new angle.
One interesting plot could be Godzilla protecting his harem from other kaiju threats. He uses his powerful atomic breath and strength to keep his mates safe.
Another interesting angle is to explore an alternate universe where Batman has been turned evil and the villains form a harem around him as a sign of loyalty. They could then go on to cause chaos in Gotham in a more organized and united front. Poison Ivy might use her plant powers to create a lair for this new 'evil' Batman and his harem, with each villain having their own area based on their skills and desires.
One interesting plot could be Batman and Sherlock teaming up to solve a mystery that involves both Gotham and London. Maybe a criminal organization is operating in both cities and they have to combine their skills. Batman's combat and detective skills in Gotham's dark alleys and Sherlock's deductive reasoning in London's Victorian - like settings would make for an exciting story.
One interesting plot could be Catwoman in the 'Batman Beyond' universe stealing a high - tech item that has the potential to disrupt the new Gotham's power grid. Batman Beyond then has to track her down, leading to a series of high - speed chases across the city's skyline. Another plot might involve Catwoman forming an unlikely alliance with Batman Beyond to take down a new criminal organization that is using advanced technology for illegal activities. And perhaps a plot where Catwoman discovers a secret about Batman Beyond's identity and uses it to her advantage in a heist.
One interesting plot could be that Batman and Lois Lane team up to solve a mystery that involves both Gotham and Metropolis. Maybe a new villain is creating chaos in both cities, and their combined skills are needed to stop it. Batman's detective skills and Lois's journalistic instincts would make for an exciting adventure.
One interesting plot could be Batman teaching Wally about stealth and detective work. Batman's dark and methodical approach would contrast with Wally's more energetic and impulsive nature.
One interesting plot could be Batman teaching Robin advanced combat techniques in a secret, underground training facility. Robin struggles at first but gradually masters the moves.
One interesting plot could be a ghoul invasion in the Batman Beyond universe. The ghouls start attacking the citizens at night, and Terry McGinnis as Batman Beyond has to figure out where they're coming from and how to stop them. Maybe there's a mad scientist behind it all, creating these ghoul - like creatures using some new technology.