I just read the Justice League comic and thought Ocean Master's characterization was perfect. He isn't a bad guy, he was just brought up with horror stories about the surface world and how evil they are to Atlantians (meanwhile Arther has lived on land his whole life.) And maybe that could be the direction of the first movie.
We start with a young Arther (small boy) living in the light house with his father. After one violent storm with waves crashing on the rocks Arther's father tells him he is afraid they will take him back. Cut to Arther as an adult, early 20s. He has always lived on land (except for maybe a brief untold period where he visited Atlantis only to return to land.) He has always known he was Atlantian, his father never hid that from him. (Maybe they could even include Dr. Shin and the idea that him being Atlantian was leaked to the press, him being a "fish boy". (Maybe we actually cut to Arther in his late 20s early 30s, and his father has already passed and he is living on land with Mera (?) and protecting the surface as Aquaman. And getting no respect from it.)
Meanwhile Orm, his half brother and some day King, finds out he has a brother who lives on land.
And thats the focus of the movie. Orm thinks he has to rescue his brother, and sends an invasion to get Arther back. Meanwhile the surface world sees it as an invasion, and Arther sees it as something he is stuck in the middle of and must stop.
Orm is shocked to find out that his brother not only loves the land but is defending it and will not return home. Orm decides to destroy the land, and forces Arther to battle. In the end Arther takes the throne from his brother and must sacrifice his life on land with his father to stop the invasion and destruction of the land.
(The Dr. Shin thing, being leaked, could lead to the surface mocking Arther and treating him like a jerk, making him the working class hero who doesn't get respect.)
Aquaman's armor chainmail shirt could be metal molded to be shark scales,
could look similar to this:
Aquaman two could be about Black Manta, a deep sea diver assassin who is sent by the surface to kill Aquaman (maybe). Just a quick thought.
I'll have to think that one out more. I like the past idea they created for him and Aquaman. I like the Dr. Shin stuff the comic has introduced, and I like how they portray Ocean Master. And I love his relationship with Mera. But I am afraid I would be adding too much.
Perhaps the first film could be about Orm, the second film could be about his rebellious pasts alluded to in the first film, his brief period in Atlantis and his connection to Black Manta.