UPDATE: Well, there it is! Just as I was working on the original story (see below) AND trying to figure out the riddles, the scene was released on YouTube! Check it out below and listen to the new BOF Social Hour where we discuss this scene.
According to Matt Reeves (via a February 2022 interview with VARIETY), the scene follows Batman’s discovery that The Riddler has killed the Gotham City police commissioner and has left another card addressed to The Batman. Seeking insight from another killer that he had taken down a year before, The Batman visits The Joker in Arkham. “Almost our anniversary, isn’t it?” The Joker asks Batman in the scene. “You realize that they have a relationship,” Reeves said, “and that this guy obviously did something, and Batman somehow got him into Arkham.”
“I thought he would be really insecure about this and he’d probably want to find some way to get into the [Riddler’s] mindset, like in MANHUNTER, said Reeves. “This idea of profiling somebody, so you can predict his next move.” – Bill “Jett” Ramey
ORIGINAL REPORT: Matt Reeves is suggesting that we head on over to RATAALADA.COM to solve some more riddles. Looks like the endgame will either be an image of Barry Keoghan as the “Unseen Arkham Prisoner,” or perhaps the entire cut scene between him and Batman will be posted. – Bill “Jett” Ramey
— Matt Reeves (@mattreevesLA) March 24, 2022