As mentioned earlier, the Fourth World mythology has already been partially explored through Batman v Superman: Dawn of Justice and Justice League, with the latter movie showing Steppenwolf leading an army of Parademons to retrieve the three Mother Boxes left on Earth and turn our world into a duplicate of Apokolips.
Darkseid was also supposed to be introduced in the movie to lay the groundwork for Justice League sequels, but in the theatrical cut, he was only mentioned once.