It's possible that the Joker kidnaps Batman to get access to the Batcave. He might be interested in Batman's technology and resources. Once he has Batman captive, he could try to trick him into revealing the location of the Batcave and then take over it, using Batman's own weapons and gadgets against Gotham.
In this fanfiction, the Joker could be planning to swap identities with Batman. After kidnapping him, he could use some elaborate scheme involving makeup and false memories to make Gotham believe that he is Batman and Batman is the Joker, causing chaos in the city.
The Joker might try to make Batman laugh at his own jokes as a form of humiliation. He could use his trademark laughing gas in a way that forces Batman to either laugh or suffocate.
One idea could be that the Joker kidnaps Batman to force him into a series of cruel 'games'. Maybe he wants Batman to choose between saving his closest allies or innocent civilians, testing his moral limits.
Perhaps they would end up turning on each other in the end. Even if they start working together, their fundamental differences would still exist. Batman's moral code would constantly clash with Joker's lack of it. So, after achieving a common short - term goal like taking down a more powerful villain or organization, they would likely go back to their usual enmity.
Well, Jay might be a character who has the ability to communicate with animals. The wings could belong to a giant bat - like creature. The Joker might try to capture this creature to use its wings for his own evil deeds. Batman would then team up with Jay to stop the Joker and protect the creature.
Perhaps it's due to a psychological breakdown. Batman has endured so much trauma over the years. The constant fight against crime, the loss of loved ones, and the never - ending battle with his inner demons could all take a toll. One day, he just snaps. His mind can no longer handle the pressure, and in an attempt to escape from his own tortured self, he subconsciously adopts the persona of the Joker, as the Joker seems to have no such moral and emotional burdens.
Perhaps Batman could develop a sort of connection to the Joker on a deeper level. Since the blood is a part of the Joker, it could be like having a piece of his essence inside. This could lead to Batman understanding the Joker's motives in a new and unexpected way. He might even be able to predict the Joker's next moves more accurately. However, it could also put Batman at risk of losing his own identity as he battles with the influence of the Joker's blood.
In a fanfic where Batman joins the Joker, it could lead to a really strange and dark alliance. Maybe Batman is trying to understand the Joker's madness from the inside. He might pretend to be on the Joker's side to stop a greater evil that only the Joker seems to know about.
If Batman becomes Joker in fanfiction, Gotham would be in chaos. Batman is the symbol of justice and order, while Joker represents chaos and anarchy. Without Batman to protect the city, criminals would run wild. And as the new 'Joker', Batman might use his intelligence and skills in a very different and dangerous way, perhaps creating more harm than good.
One possible plot could be that the Joker kidnaps the girl to use her as a bargaining chip against Batman. He might think that Batman has a soft spot for this girl and would do anything to save her. So, he holds her in a secret location filled with his signature traps and chaos.
One thing that could happen is that Keiko tries to talk her way out of the situation. She could try to appeal to Karasa's sense of humanity or reason. For example, she might say that she has a family who will be worried about her or that Karasa will get into a lot of trouble if he doesn't let her go.
If the Joker is captured by Batman in the fanfic, Batman could use this opportunity to study the Joker more closely. He might try to understand what really makes the Joker tick, his origin, and his motives in a deeper way. However, the Joker might be planning an escape even while being held captive. He could be using his time in captivity to set up a grander scheme that would put Gotham in even more danger once he breaks free, all while constantly playing mind games with Batman.