So happy James Wan is embracing the classic looks and attempting to show audiences why they are cool rather than change them... *cough Snyder cough*
He's said the inspiration is mostly the New 52 design, which is hardly classic, though it doesn't deviate too much from the classic costume; the armor near the back seems new to me. It's also been said that Arthur likely won't wear that costume until near the end when he fulfills his destiny. First, as the plot details have hinted, he will have to find the trident, which in this story sounds kind of like excalibur, and reconcile himself with his Atlantean heritage. Superman in
Man of Steel didn't start wearing his costume, which was also mostly classic and New 52 inspired sans collar, until he had spent years seeking out information about his alien heritage and embracing both the crest of the House of El and the destiny Jor-El told him about. These characters are on heroic journeys, and their costumes are a part of that.
Or since Arthur Curry wasn't the king of Atlantis by the time Justice League (and BvS) happened he decided it was up to the next director to give his own spin to Aquaman's original costume (and give him his trident).
Exactly. It's something Snyder did for Barry as well. He confirmed on Vero that he wanted to save Barry's new suit for the Flash solo. I believe the same thing happened for Arthur in JL. The quindent that Arthur uses once belonged to his mother, and it, along with the armor used in JL were, I believe, given to him to use by Mera and/or Vulko (Defoe) whose scenes were cut from the Whedon version.
Aquaman picks up after this, as we get to watch Arthur accept and earn the iconic look.