With both the Batman and Aquaman movies delayed, Warner Bros. is looking to fast-track another DC film to begin shooting this year, according to Variety's Justin Kroll. Although unconfirmed (and especially suspect given Kroll tweeted out this report rather than writing it for Variety), Warner Bros. is said to be looking at Gotham City Sirens, The Flash, Green Lantern Corps, Dark Universe, or the untitled Suicide Squad sequel to begin filming in 2017.
"That said if scripts aren't ready to go, studio is content just shooting Aquaman this year till films are ready to go."
Only Gotham City Sirens and Dark Universe have confirmed directors - David Ayers and Doug Liman, respectively.
Last Thursday, it was reported that Aquaman was pushed back by Warner Bros. from October 5, 2018 to December 21, 2018, following the the latter date being freed up by the delay to James Cameron's long-simmering Avatar 2.
The Flash was originally scheduled to begin filming in 2017 for a March 2018 release, but the departure of its third director led to reports of delays which have yet to be confirmed.