Recently, job hirings have shed some light on when Rocksteady plan to announce their next big AAA title, alongside an industry insider letting it slip that contingent upon some milestones, Rocksteady could announce their game in early 2019.
Now, a new 4chan post might have shed some light on what their new game is.
I'll copy and paste the bullet points so it doesn't get too convoluted. Here's what it all says:
Possible titles - Justice League: Crisis, Justice League: New Crisis, Justice League: Infinite Crisis. Seems they haven't settled on a title yet, but they have a new staple in Crisis. Kind of like how Arkham was to their Batman series.
Developer - Rocksteady
Systems - Xbox Scarlett/PS5/PC. You'll hear more details on Xbox's new system later this year with a full reveal early 2019. PS5 will be announced in early 2019 as well.
Type - Single Player/Co-op. You can switch between characters like in Arkham Knight. The campaign can be played both ways, but there's also a different game mode catered to multiplayer. Also, it's GaaS, so expect a ton of DLC content. They're gonna have something similar to Hitman with episodic DLC.
Characters - Superman, Batman, Wonder Woman, Flash(Pretty sure it's Wally), Green Lantern(Hal), Cyborg, Aquaman.
Cities - Metropolis, Gotham, Keystone(why I said it's probably Wally), more others through DLC. Each city is a bit bigger than AK, and just as detailed.
Villains - Main big bad is Starro. As the episodic stuff rolls out you'll see Brainiac and even Darkseid.
Story - All the heroes are pretty early on in their careers. Batman's still considered an urban legend to anyone outside of Gotham, Superman isn't the "world protector" yet, and WW is still on Themyscira. Aquaman's still in the sea, GL is in space, and Wally isn't even a hero yet.
Gameplay - From what I hear, it plays just like the Arkham games, except more refined and "opened up". Guessing that's for the heroes that can fly and such.
DLC - A lot of the DLC will have new cities and new playable characters. Green Arrow, Hawkgirl, Martian Manhunter and probably a lot more. They're expecting this game to last for years, so I wouldn't be surprised if this game ended up being like Destiny in the sense that it gets updated constantly.
At first glance this post might seem derivative of other Rocksteady posts, but something sticks out...
They say the main villain in the game is Starro. Sounds kind of lame, until you look back at the clues Rocksteady has hidden in previous games. In all of their past games, Rocksteady has laid an easter egg that points to their next game. In Arkham Asylum we had the secret room. In Arkham City we had the scarecrow barge, and I believe the last one is in Arkham Knight.
In Arkham Knight's DLC, Batgirl: A Matter of Family, there's a hidden Starro Easter Egg where you can actually see the villain hidden within a gigantic test tube! Here's a link to a video showing it:
https://www.youtube.com/watch?v=hmGIvs7IxJA&t=13s
Couple that with Rocksteady's recent job hirings (multiplayer testers, network coders, specialized GaaS managers) and the fact that they were specifically hiring for "next gen" and for people with "broad comic book knowledge" and I think it's safe to say they are indeed working on Justice League for next-gen.
EDIT: Also forgot to mention that with the possible titles: justiceleaguecrisis.com is owned by WB, and was recently updated this year.
UPDATE!: The mods over at /r/BatmanArkham quietly took down the post. All of the other times they would say "Posts like these should go under the rumor/leaks thread" and then move it, but instead they took this one down. I think there's some truth in here.