Despite constant rumours that the character would be making an appearance in Batman v Superman: Dawn of Justice (which of course proved unfounded), it seems that Warner Bros. had no plans to utilise Barbara Gordon, a.k.a. Batgirl, until Joss Whedon convinced them otherwise.
In an article on the status of Warner’s DC film division(s), Variety reports that the Batgirl movie is “one that the studio hadn’t planned on doing until Whedon confessed to his love of Barbara Gordon and her masked alter ego.”
The Batgirl movie was announced back in March, although Whedon presumably put development on hiatus after boarding the Justice League movie following Zack Snyder’s departure for personal reasons. DC’s Geoff Johns has since said that Whedon will begin work on the movie in 2018. Details are scarce, but it is thought that it will be based upon the New 52 version of the character.