One interesting story could be where Joker captures Batman and tries to break his spirit by showing him a distorted Gotham City that he has created in an abandoned area. Joker might use his henchmen to taunt Batman while he's trapped, but Batman uses his wits to find a way out and turn the situation around on Joker.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of Batman - related fanfictions on it, and you can easily search for ones where Joker captures Batman by using the search bar and typing in relevant keywords.
One idea could be that Batman captures Percy because he mistakes Percy's powers for something dangerous. Maybe he sees Percy using his water - based abilities in Gotham and thinks he's a new super - villain. Batman then has to figure out that Percy is actually from a different world and not a threat to Gotham.
At first, Batman would be shocked and a bit confused. But quickly, his detective instincts would kick in. He would view the deaged Joker as a new and unexpected threat. He'd start gathering information about how the deaging occurred, in case it was part of a larger, more sinister plan. Batman would also be very careful not to underestimate the deaged Joker. He might try to isolate the Joker in a secure location to prevent him from causing chaos in Gotham. And he'd be constantly on the lookout for any signs of the Joker's old, devious self emerging from this new, younger form.
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.
Some writers explore their relationship through a cat - and - mouse game. Batman is always chasing the Joker, and the Joker enjoys taunting Batman. It's like a never - ending dance. For example, the Joker might leave clues for Batman just to see if he can solve them, and Batman is always one step behind but determined to catch the Joker.
Next, Percy could use his wits to escape Batman's clutches. Since Percy has experience dealing with various gods and monsters, he might be able to outsmart Batman. He could create a distraction using his powers, like summoning a small flood in the Batcave, and then make his getaway.
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.
A really interesting fanfiction is 'Joker's Gambit'. In this one, the Joker sets up an elaborate scheme that not only challenges Batman physically but also mentally. It's full of unexpected twists and turns that keep you on the edge of your seat. The author does a wonderful job of portraying the cat - and - mouse dynamic between the two iconic characters.