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.
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.
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.
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.
In the fanfiction, Batman might adopt the little girl because she shows extraordinary intelligence or a unique ability. For example, she could be a tech prodigy. This would attract Batman as he always looks for ways to enhance his crime - fighting capabilities. He would then have to balance his role as a father figure with his duties as Batman, which could lead to some interesting internal and external conflicts.
Well, it could be that Batman sees Spiderman as a young hero in need of proper guidance. He might train Spiderman in more stealthy combat techniques, as Batman is known for his stealth. And he could also introduce Spiderman to his high - tech gadgets and resources, which would enhance Spiderman's crime - fighting abilities.
Wally is another key character. He has super - speed which makes him a very dynamic and exciting character. In this fanfiction, he is in a position to be mentored by Batman, which will likely lead to a lot of character growth as he learns from Batman's experience and discipline.
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.
Obviously, Batman and Jason are the main characters. Batman is the mentor and adoptive father figure, while Jason is the young boy who is being taken under Batman's care.
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.