The story could explore how Marinette adapts to Gotham. With her skills as Ladybug, she could have some unique abilities to offer in Batman's city. Batman would probably be very strict in his parenting style, but also protective. He could teach her advanced combat and detective skills. And Marinette could introduce Batman to the more magical side of things, as she has experience with the Miraculous. It would be an interesting blend of two different universes.
One possibility is that Marinette's family situation in Paris somehow leads to Batman adopting her. Maybe there was a threat from Hawkmoth that was too big for her to handle alone, and Batman steps in. He would then have to deal with her dual identity as Ladybug. He might try to keep her Ladybug activities a secret from the rest of Gotham, while also helping her to become a better superhero. It could be a story of growth, for both Marinette and Batman, as they learn from each other.
For Marinette, she might become more serious and less impulsive. Growing up under Batman's guidance, she would learn to think more like a detective and strategist. For Batman, he could become more open - minded. Marinette's positive attitude and creativity could break through his usual stoicism.
Well, perhaps Batman sees Superboy as a lost and powerful youth in need of guidance. He could take Superboy under his wing to teach him how to control his powers better and use them for good. Maybe they start by training in the Batcave, with Batman showing Superboy his high - tech gadgets and combat techniques.
One possible storyline could be that Batman sees the potential in Shazam, who is still a bit of a naive hero. Batman takes him under his wing to teach him the more strategic and stealthy aspects of crime - fighting. For example, Shazam might be used to just using his superpowers to barge into situations, but Batman could show him how to gather intel first.
One reason could be the exploration of Batman's softer side. When he adopts Ace, it shows his capacity for love and family outside of his crime - fighting duties. It gives a new dimension to his character.
It's possible that the little girl's presence would soften Batman's tough exterior. He's always been this brooding, dark figure, but having a daughter - like figure in his life could make him more approachable and show a different side of him to the people of Gotham.
A batman and spiderman crossover fanfic could be about them teaming up to fight a common enemy like the Joker and Green Goblin who have formed an alliance. They would combine their unique skills, Batman's detective skills and martial arts with Spiderman's agility and web - slinging.
Imagine Batman on a stage, with a spotlight on him, singing fanfic. His cape flowing as he belts out the words. It could be a mix of his dark and serious nature with the creativity of fanfic. He might choose to sing fanfic that focuses on his heroic deeds, making it like a ballad of his adventures. Or perhaps he'd sing about his inner thoughts and struggles, giving fans a deeper look into his complex psyche. This new ability could even lead to new storylines where his singing becomes a key element, whether it's to inspire others or to use as a distraction during a fight.
Well, in the main canon, Batman doesn't usually have this kind of direct family - like relationship with Ace. So the fanfic is a divergence from the norm.
Darkseid could be a potential main antagonist. He might view Superboy as a weapon that he wants to control. If he realizes that Batman is trying to shape Superboy into a hero of Earth, he would surely try to stop it. He could send his minions to attack Batman and Superboy or try to corrupt Superboy's mind from afar with his Omega Beams.
Well, they might start by simply petting the kittens. Maybe the kittens' soft fur and cute meows would make them forget about all their daily troubles. Adrien could be sharing some of his modeling experiences while Marinette shyly listens and giggles. They could also play with the kittens using little toys, like a ball of yarn, and have a really good laugh as the kittens chase after it.