According to ScreenRant, Warner Bros. and DC Entertainment are developing a film about Harley Quinn and the Joker, both of whom starred in Suicide Squad. The project would obviously be part of the DC film universe and would be linked to Suicide Squad, and the project Gotham City Sirens, scheduled for next year.
The title of the project would be Harley Quinn vs. The Joker. As ScreenRant claims, this new film would be considered a spin-off, with Geoff Johns producing.
Note: during the Comic-Con this weekend, Warner Bros. announced the release of two films for 2020, without specifying their names nor giving indications as to the characters considered. Until then, fans will have to enjoy the upcoming Justice League, out November 17.