Reeves' thoughts on the Joker appearance:

In an interview with Variety, Reeves spoke about his actual intent with the cameo. Not exactly meant as a sequel hook, but rather as a way to further establish Gotham City as its own character, the appearance of The (almost) Joker is still an important piece of the puzzle. "I thought it'd be really neat if so much of the fabric of Gotham just already existed," Reeves explained. "And it was like an old Warner Bros. gangster movie and if you took a certain turn, you might see a character in his origins." This effectively turns the film into a more immersive experience for those looking to really dive into Gotham as a setting.

But that doesn't mean he has plans to use that particular element going forward. "It's not an Easter egg scene," Reeves said of the scene itself. "It's not one of those end credits Marvel or DC scenes where it’s going, like, 'Hey, here's the next movie!' In fact, I have no idea when or if we would return to that character in the movies." That's sure to be a bit of a blow to eager fans whose excitement levels rose at the prospect of a full movie starring Keoghan as the Clown Prince of Crime.
He IS in jail in the movie. This makes you think that Batman already caught him at some point during his first two years. Let's hope he stays there for a long time. Wouldn't want to see him until a 4th or 5th movie in this universe.