DCEU: 9 Things We Learned From 4Chan's Latest DC Movies "Leak"
3. Gotham City Sirens Or Justice League Dark For 2018

The Rumour
Warner Bros. would ideally like a second film for next year, with Gotham City Sirens the top choice, and Joel Edgerton being eyed for the role of Black Mask. Warners. would want to start shooting around August. The other possibility is Justice League Dark, which apparently has a finished script, and several directors being eyed after Doug Liman stepped away.
Response
Aquaman isn't due to be released until December 21, 2018, which would mean a wait of 13 months before DCEU films. That's not the kind of gaps studio execs want in a shared universe - hence why we had two DCEU movies last year, and two this year - so it absolutely makes sense Warners would want a second one to fill the gap. This also fits with reports from March, when five movies were in the running to be filmed this year.
I wrote then that Gotham City Sirens and Justice League Dark were the most likely two, so that also fits, but it's going to be a hell of a task to have either of them ready to go soon. JLD has just lost its director and doesn't have a cast, while Sirens has a director (David Ayer) and a writer (Geneva Robertson-Dworet), but hasn't yet cast Catwoman nor Poison Ivy. You'd imagine they'll both be big(ish) names.
Neither are outside the realms of possibility, and Sirens is the more likely of the two, but we'd need some casting news very soon.