I was really, really hoping it would be Joaquin Phoenix's Joker. I know that in terms of timelines it wouldn't really make sense since Joker took place during like, the eighties- Unless we retcon it so that he just remembered himself being his current age when telling the story to his therapist- But I still just would have loved to see him. When he said "One day you're on top, the next day you're a clown" I was hoping for him to go into a line from "That's life." Would have been perfect.

Granted I don't think it's going to be that The Joker will play the main villain in one of the Reeves Batman movies in the future. I think it's more likely that he'll be a supporting cast member in a large and growing number of crazy people in Gotham. This kind of vibe, where whenever Batman has trouble you can expect The Joker to be following it around.