Some superhero movies require more setup work than others. A Batman movie has to introduce one hero, his butler, assorted Gotham locals, the villain, and maybe a sidekick. Aquaman has all that and seven kingdoms worth of politics, war, history, and strife. Someone could make a Game of Thrones series of Atlantis and its six siblings. King Nereus appears in the film and will return for the sequel, but he's a slightly different character in the DCEU.