He could be initially shocked. But then, being the Joker, he might turn it into some sort of twisted game. Maybe he starts using his revealed identity to create new chaos. For example, he could use his real name to frame innocent people or create false identities for himself to further deceive the public and Batman. He might also see it as a new challenge from Batman and try to come up with even more elaborate and dangerous schemes.
The Joker might be furious. His identity has been his biggest secret and Batman taking that away is like stripping him of his power. He could go on a wild rampage, more so than usual, targeting Batman directly and also anyone who he thinks might have had a part in the revelation.
One interesting plotline could be the reaction of Gotham's criminals. They might either be overconfident thinking they can now easily target Batman or be terrified knowing his true identity means he has more resources at his disposal. Another could be how Batman's allies, like Alfred, deal with the new situation. Alfred might be even more protective or start to worry about the safety of Bruce Wayne's loved ones.
In fanfiction, when Harry Potter reveals himself, it could be a moment of great drama. Maybe he reveals his true identity to save his friends from a dangerous enemy. For example, if they are being tricked by a Death Eater in disguise, Harry might step forward and expose himself to stop the harm.
Batman's allies like Robin (whichever version - Dick Grayson, Jason Todd etc.) are often involved. They might have to deal with the fact that their mentor's identity is now public and face new threats because of it. Also, Commissioner Gordon is a main character. His relationship with Batman has always been complex, and this revelation will surely change how they interact.
Well, Arthur could feel betrayed at first. He has been brought up to believe that magic is bad, and here his closest friend has been hiding this big secret. However, as he realizes how Merlin has saved his life and the kingdom many times with his magic, he might come to accept it and even see the potential of magic being used for the good of Camelot. For example, Merlin's magic has defeated many enemies secretly, which Arthur might now understand as a necessary protection for the kingdom.
Well, Gordon's reaction could be complex. On one hand, he might feel honored that Batman has chosen to disclose his identity to him. On the other hand, he could also be worried about the implications. Knowing Batman's identity means he has a greater responsibility to protect that secret. He might start to think about how this new knowledge will affect their working relationship and the safety of Gotham. For example, if the wrong people find out that he knows Batman's identity, it could put him and his family in danger.
It could cause chaos in the underworld. Once the Joker's identity is revealed, his associates might feel betrayed or threatened. Some might try to eliminate him to protect themselves from being exposed as well. Also, other villains in Gotham might see this as an opportunity to take over the Joker's territory or operations, leading to power struggles and more crime and violence.
Batman would probably first try to contain the situation. He'd use his resources as Bruce Wayne to spread misinformation or counter the revelation. He might also go on the offensive against Scarecrow to stop him from spreading the news further.
In the fanfiction, Naruto's enemies' reactions could vary greatly. For instance, Orochimaru might see it as a chance to conduct more experiments on Naruto or the tailed beast. The Akatsuki, on the other hand, might change their plans completely. They were after the tailed beasts, and now that Naruto has revealed his, they might either try to capture him right away or come up with a more elaborate plan. And then there are smaller enemies who might just cower in fear at the thought of facing a jinchuriki like Naruto who has openly shown his power.
Some characters might be amused. They could find the creative interpretations in the fanfiction interesting, especially if it gives a new spin on their relationships or adventures in the Batman universe.
In some fanfictions, when Woody reveals himself to Andy, it could be a very emotional reunion. Maybe Woody has been lost or on an adventure far away, and his return brings back all the great memories they had together. Andy might be overjoyed to see his old friend again and they could start a new adventure right in their own backyard.