I have a couple thoughts:
1. It's not going to happen. This is just like the Green Arrow Supermax movie. Or any number of projects that studios have had pitched to them or even scripts that have been written. Add to that, I read an interview with Del Toro saying he loved the idea but hadn't heard of it yet. I don't know if studios leak it intentionally, to gauge reaction, or if it is people working there who leak it or what.
2. If, and the odds are seriously against it, it DID happen it would not have a Justice League title. THe brand is beingbuilt for Justice League proper, and another Justicee League title -- especially a "dark" title with C-listers, and "horror" C-listers at that -- would dilute the brand FAR more than the Hulk movies ever did.
3. From a purely logistical standpoint, this movie would be far too difficult to mount and to write. Justice League proper can work as a movie without much lead up because the characters are in the public conscious. If you ask teh average person who Aquaman is, they can at least say "water superhero", probably will say "talks to fish", and might even say "wears orange." Superman? "Rocketed to earth as a child, now can fly." People know that. Batman . . . "parents murdered, now he fights crime." Flash: "runs fast."
Spectre? Deadman? The only character there with any public knowledge is Swamp Thing, known for two camp movies and a TV series and cartoon. But he's not going to carry his own movie these days. The movie would have to introduce SEVEN (or however many) characters to people who have never heard of all but one of them. Even Avengers had to make enough movies to count as a television season to introduce the characters.
4. On a similar note, all of these characters are rich enough to deserve their own movie. But the ONLY way I could see this happening would be if it was a John Constantine movie, where he was on a mission that used these characters as incidental characters. Maybe he's building an army or something. John Constantine would make for a good movie too. Oh, wait . . .
So, as much as I love Del Toro, and as much as I love Swamp Thing, and as interesting as those other characters are, I think we're safe. This movie will not happen, and therefore will not bomb.