Zack Snyder has told Empire that Superman’s role in Justice League will be larger than people expect, but didn’t specify whether this was due to Superman’s presence or absence. Both of these would be relevant, given how much of an effect Superman has had on the world. We see in Batman v Superman: Dawn of Justice that Superman has his critics and followers, but Suicide Squad showed that his loss is mourned. Does this mean that Superman will be in a Luke Skywalker scenario – only appearing at the end, in order to lead-into a sequel? The Justice League are being assembled in order to defend a world without Superman, and Superman having any involvement in that would somewhat contradict the point of their existence.
Personally, I think that the information Superman obtained before his death has prompted him to travel to Apokalipse in order to fight Darkseid. Maybe he returns to Earth as a brainwashed agent?