So there were two scenes with the Joker initially. Reeves told the French press about it here:
The Batman : la surprise du film expliquée par le réalisateur Matt Reeves
Translation of Reeves' statement:
“Does the film introduce us to the main antagonist of the next film? No, that was not the intention”, affirms Matt Reeves to our microphone. "I don't even know who the villain will be in the sequel. The character was supposed to appear earlier, in another scene, but it's all about the context: I didn't want to do a Batman origin story, because I had the feeling that it had been done, and very well, in several other films."
"Instead, I wanted to show him in his younger years as a vigilante. Like a 'Batman: Year Two'. And as I delved into the comic books, I discovered that a lot of his enemies emerged as a reaction to the presence of this masked individual called Batman in Gotham, and I realized that's where their origin story lay. "
"This character in Arkham goes back to the Joker before he was the Joker. He hasn't decided to claim that concept yet. But I wanted to make him someone that Batman met in his first year, and that he had him locked up because he was a killer. And because Batman was disturbed by the Riddler writing to him, he had to go to Arkham to try to establish his profile, see if he could succeed in entering his state of mind to understand the reasons for these letters."
"When he goes to the asylum, the Joker who is locked up - but who is not the Joker because there is none - manages to read in Batman: 'Why do you wonder why he's writing to you ? You are exactly the same, both masked vigilantes.' He draws a comparison between the two, and Batman is so pissed off by the idea that he rejects it."
"The scene was originally in the movie, and Barry Keoghan was great, as was Robert Pattinson. But seeing how big the movie is, I ended up realizing it didn't need to be there. I Still kept his second scene. Because it marked the end of the Riddler arc, but also because it showed that more trouble was coming."
"When Selina tells Bruce in the epilogue that the city will never change, removing the scene [with the future Joker] changed the stakes. Because you didn't have the feeling that something was already brewing. Even if the intention wasn't to say: 'This is where the next film will go!' The idea was more to give some context to this world, and to show that even if the stranglehold of corruption has been broken to some degree, trouble is not going to stop coming."
"And that explains in part why Bruce doesn't go with Selina. Why he can't. He is forced to do what he does, and that's why I saved this scene. Anyway, all this to say that this character is the one you're thinking of, but that doesn't necessarily mean he'll be the next movie's villain."
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I have to say I don't find that justification for keeping only this second scene very convincing. The movie ends showing how a part of the citizen had quickly followed a murderous psychopath, we know that Oz is becoming the new crime lord and obviously we're not gullible enough to think you can stop corruption and crime just by taking some people off...
I wonder if WB asked him to kept at least one of the two when he decide it hasn't really its place in the movie. I would have deleted both or just kept the other aha. On paper, it just seems so much more relevant to the plot in several ways. Also, I would have been curious to see Keohgan in this one, as I found the little snippet we got rather sterile
(but like I said, I won't condemn his casting, the apparition was too short to be fairly judged).