When Willem Dafoe was cast in a mystery role in Zack Snyder's Justice League, I asked our commenters to give us their suggestions and guesses as to who he might be playing in the movie. The one piece of information we had was that he was said to be playing a good guy, but a majority of our commenters thought this was misdirection; turns out a lot of people really want to see Dafoe as The Joker in some form or another. (The continuity gets a little tricky with that one.) Others took the "good guy" news at face value and suggested the Martian Manhunter (a great suggestion).
BirthMoviesDeath has done some digging, and though they didn't get a specific character name, they did find out that Dafoe will be playing an Atlantean in the two-part superhero saga. Who could that be? Seems like the right age for Arthur Curry's father, but when you look at it, it seems a little preposterous that Dafoe could ever be Jason Momoa's dad.
Additionally, the site says that the Aquaman solo movie is undergoing a page one rewrite, and that new technology is being implemented in Justice League (and presumably Aquaman) to make the underwater scenes look better than they did in the brief cameo in Batman v Superman: Dawn of Justice.
In Batman v Superman: Dawn of Justice Aquaman's cameo was marred by the fact that Jason Momoa was very, very clearly holding his breath in a water tank. Like, his cheeks are all puffy as if he were chilling on the bottom of the pool at his parents' house. It's not a good look for an amphibious hero. My sources tell me that rather than shoot in a water tank this time Zack Snyder has set up an elaborate rig on a green screen stage - it's a two armed rig, with Momoa clutched in one arm while the camera is clutched in the other. They two arms are preprogrammed to move in sync so that Momoa can do all kinds of maneuvers while the camera moves around him. I'm assuming there's a similar rig for more elaborate flying stuff. Momoa's hair will be CGIed into a flowy, underwater look.
Sounds cool to me, and Snyder certainly has a flair for visuals. If Aquaman is playing a larger role here, it stands to reason that he'd devote more time to making sure his scenes look better than they did before.