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.
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.
Robin does not become the Joker. The characteristics and roles of these two characters are completely different. They exist within separate story arcs and have no direct transformation connection.
There's also 'Batman: Under the Red Hood' which, while not directly about the initial 'Joker kills Robin' event, still touches on the aftermath. It shows how the death of Robin haunts Batman and how it affects the actions of other characters in the Batman universe. It also gives more insight into the Joker's psyche and his role in Robin's death.
If Karasa kidnaps Keiko in a fanfiction, the story could go in many directions. For instance, Keiko might turn out to be much stronger than Karasa expected. She could use her wits and any special abilities she has to try and escape. Or perhaps there are other characters who will come to Keiko's rescue. They could be friends, family, or even strangers who happen to be in the right place at the right time.
In many Batman and Robin fanfictions where Robin is injured, it often leads to Batman becoming more determined to catch the villain. For example, in some stories, Robin gets injured during a fight with a new, powerful enemy in Gotham. Batman then uses his detective skills to track down the bad guy while also taking care of Robin. He might call in Alfred for help with Robin's injuries. This kind of situation shows the deep bond between Batman and Robin and how they rely on each other in tough times.
It made the Batman universe much darker. The death of Robin was a huge shock, and it changed Batman's character. He became more brooding and intense. It also showed how dangerous the Joker really was, not just as a villain who commits crimes, but as one who can cause such personal and emotional harm to Batman.
Robin would probably be very hurt. He might initially go into a state of shock and disbelief. After that, he could try to find evidence to prove his innocence. For instance, he might search for surveillance footage or eyewitnesses.
In a Shego - kidnaps - Kim - Possible fanfiction, the plot could be filled with high - stakes action. Shego might have some devious plan behind the kidnapping, perhaps related to a powerful artifact Kim was about to retrieve. Kim, being the resourceful hero, would surely find ways to outwit Shego even in captivity. This could lead to an exciting cat - and - mouse game within the story.