While the script has only just been started according to screenwriter Charles Roven, rumors are already flying about Aquaman, the 2018 film in the new and growing DC Comics movie universe. JoBlo has a rumor that the early talks about the story have a villain: Ocean Master.
The character is Aquaman's half-brother, and thinks himself the true heir to the throne of Atlantis. While JoBlo frames this as a surprise villain, it's actually fairly obvious if you've looked at the media of the last few years. Ocean Master, or Orm, has been a primary villain for the entire newest run of the Aquaman comic books. He was also featured as the main villain in the animated film, Throne of Atlantis.
Which brings up an interesting point: the current rumored villain and storyline for Aquaman being Orm fighting for control of the throne sounds like that animated film. The rumor of Red Hood being the villain and storyline for Batman (Affleck edition) also reflects an animated film from a couple of years ago. Could the animated DC movies be a proving ground of sorts for characters and storylines?
JoBlo also says that Mera could be cast with a non-white actress, but didn't have any other details or specifics.