I like the inclusion of a Barry Keoghan Joker in the proceedings of this film but I think there may have been a more seamless integration already baked into the story that the filmmakers could have capitalized on:
Keoghan should have played the funeral-goer that Bruce Wayne sees at Don Mitchell’s memorial—the man who cozies up to Wayne to share his disdain for the late mayor and the city’s elite.
This man is only later seen again as part of the assault against Mayor Real at Gotham City Garden. He’s one of the Riddler’s army—one of 500 nameless Gothamites. This universe’s Red Hood Gang (or green mask gang, as it were).
At the GCG, this man nearly gets the drop on Batman but is stopped by Selina who he in turn nearly kills. Batman is barely able to subdue him.
When Gordon arrives and asks the man his identity, he responds among the choked laughter… “I’m vengeance,” which causes Batman to remember his face.
The Riddler is the match to this city’s powder keg…
The nameless man laughs maniacally and sets an explosion/throws a grenade which causes him to fall into the water as a dangling wire sparks with electricity.
Instead of searching for this random Riddler goon’s body, Batman goes to save the mayor’s son and mayor-elect Bella Real from the wreckage and lead them out of the Garden.
The end of the film could have a quiet scene of this nameless man, now reborn as Joker, shrouded in shadow and smoke, surveying the chaos of No Man’s Land Gotham City as his laughter echoes into the transition to the film’s final scene between Selina and Batman in the graveyard.