Well I'm a little late but I want to go back to the two justice league movies. One thing I agree with is the idea that Darkseid can carry both movies but you don't need to have the first build up to him, just have the first be similar to war, different Darkseid portrayal have the justice league be at odds with different government programs while having this mystery story of people disappearing and then reveal that apokolips has been manipulating the world behind the scenes preparing the invasion. You have political and spiritual conflicts going on for example some could believe this is the biblical apocalypse creating a truly apocalyptic end of the world scenario. Darkseid arrives leveling Gotham recreating batman's nightmare. The league make their last stand against Darkseid resulting in an epic battle the league manage to win and save the Earth but have no idea what Darkseid sought. That's just part 1, sounds like a lot though so does dawn of justice.
Part 2 then introduces Orion who could show up at the end of part 1 we then get the full New god's backstory leading to a full on cosmic epic, a full on War of the gods that the league are caught between. Have Darkseid tear apart the league and even level new Genesis, at the end The league and Orion battle a near all powerful Darkseid, in the end Orion kills him by ripping out his hear. Later down the line you can reveal that Darkseid and the remaining new god's were all reborn on Earth setting up Darkseid getting the equation and final crisis somewhere down the line.
Basically part 1 is the classic Darkseid invasion while also giving glimpses of the new god mythology, part 2 is just as much a new god's movie as it is a justice league think JL twilight part 1 and 2. A two part epic telling the story of the fourth world, it would be massive and truly climatic.