Joker: 5 Theories That Actually Improve It
4. Robert Pattinson's Batman Will Appear in a Sequel
The movie ends with the murder of Bruce Wayne's parents as they leave the theatre, a sight that we’re rarely treated to in a Batman (related) movie…
This certainly sets up a Batman in this universe and if, as Phoenix and Phillips have both teased, there is potential for a sequel, it will surely involve the Caped Crusader. Batman is such an integral part of the Joker character, even more so now that Joker's actions directly sparked the event that leads Bruce Wayne on the journey to becoming Batman.
Additionally, Matt Reeves' 'the Batman' is rumoured to be set in the 1990s, so the timelines would line up smoothly, with Joker being set in 1981. Whether or not this will be part of the DCEU is still up for debate, perhaps this new Joker/Batman universe will remain self contained.
Some could argue that, if Batman was to exist in this universe, there would be a significant age difference between Batman and Joker. In the movie, Joker is aged somewhere between 35 (at best) and 45. This means that, as Bruce Wayne is around 10 in the movie, Joker would be around 60 years old by the time Batman shows up.
That being said, this age gap wouldn't be much of a problem in terms of plot. In Batman 1989, there was a 30 year age gap between hero and villain. Furthermore, there is no real need for a physical match between the two. Batman has always easily bested Joker in a fight, the true draw to Joker is his schemes.