In fanfiction, once Joker finds out Batman's identity, the development is quite intense. The story could see Joker infiltrating Bruce Wayne's business operations. He might try to bankrupt Wayne Enterprises to cut off Batman's resources. Also, he could use the knowledge to blackmail Bruce, forcing him to make difficult choices between his life as a billionaire and his role as Batman. Joker would also likely increase his violent activities, knowing he has the power to really hurt Batman on a personal level.
The story often takes a dark turn. With Joker knowing Batman's identity, he can directly attack Batman's weaknesses. For example, if Bruce Wayne has a love interest, Joker might target her. This creates a new level of tension and danger in the story.
Often, the story becomes a race against time. Batman has to find a way to protect his identity from being exploited further. He may have to deal with mobs of people, some who idolize him and some who want to harm him.
If Joker finds out Batman's identity in fanfiction, it could lead to a total breakdown of Batman's world. Joker might target Bruce Wayne's loved ones directly. He could expose Batman's identity to the public, causing chaos in Gotham as the people lose their faith in the symbol of Batman. And he would likely use this knowledge to create the most elaborate and cruel traps for Batman, knowing his true self.
One possible consequence is that Batman's loved ones would be in great danger. His enemies could target them to get revenge on Batman. Also, the public's perception of Batman might change. Some might see him as a hero who has been hiding his true self to protect them, while others could think he has been deceiving them all along.
In 'Batman: The Dark Knight', the Joker develops from being just a criminal nuisance to a real threat to Gotham's very existence. His actions become more extreme, like when he kills important people. He also shows his ability to manipulate people's minds, which grows throughout the story.
The story could develop in a very dark way. Batman, as Joker, might start to create his own brand of 'jokes' which are actually cruel and harmful pranks on the citizens of Gotham. His rogues' gallery might be both confused and excited by this change, not sure whether to team up with him or oppose him.
When Harry is raised by the Joker in fanfiction, his character development would be quite extreme. He'd start to embrace the Joker's values of chaos and anarchy. He might become desensitized to violence and suffering, seeing them as means to an end - that end being entertainment or causing disruption. His relationships would be based on power and manipulation rather than love or friendship. He'd constantly be looking for ways to outwit others and cause mayhem, losing any sense of innocence or goodness he might have had originally.
The story could develop with Batman trying to find a cure. He'd be desperate to get rid of the Joker's blood in his system. He might seek out the help of Alfred or some of his scientific contacts in the DC universe.
The Joker in the Batman New 52 story evolves in a complex manner. His character is more unhinged than ever. He shows a greater understanding of Batman's weaknesses and uses them to his advantage. For instance, he might target Batman's friends or places that hold sentimental value for Batman. His evolution also includes a more grotesque and disturbing aesthetic. His makeup and overall look seem to be a reflection of his inner state of mind. He is constantly pushing the boundaries of what is considered acceptable behavior, even for a villain in Gotham. His actions have a wider impact on the city, causing not only physical damage but also psychological trauma to the citizens.
It can completely change it. Usually, there's this cat - and - mouse game between them. But when Batman hurts the Joker in fanfiction, it might make the Joker more vengeful. He could become even more obsessed with getting back at Batman.
Batman becomes more brooding. He's less likely to trust others and more likely to take on dangerous tasks alone.