Back when Aquaman’s look was first revealed ahead of Batman V Superman: Dawn of Justice, his solo poster simply read “Unite the Seven.”
The colored sky around the Justice League here was also on display in the movie’s official trailer, with each hero getting a quick second to prepare for whatever they are looking at.
That did not stick and can now be looked at as simply a reference to Aquaman uniting the seven seas.
However, since this version simply (and smartly) doesn’t put a number on the League’s roster size, the possible “surprise” appearances of Superman or even a Green Lantern may yet happen.