Warner Bros. Animation's next DC film will be an adaptation of the 1989 one-shot Batman: Gotham by Gaslight, according to the back cover to the Batman and Harley Quinn home video release packaging solicited on Amazon.
Written by Brian Augustyn and drawn by Mike Mignola, Batman: Gotham by Gaslight is arguably the first "Elseworlds" story done at DC Comics. Set in the late 1800s, Bruce Wayne/Batman goes up against Jack the Ripper. Augustyn wrote a sequel several years later titled Master of the Future with artist Eduardo Barreto.
No release date for Batman: Gotham by Gaslight has been given.