Well to begin with this i think that darkseid would have entered the world of DCCU already and that he would have conqured the earth. He would be mindcontrolling superman or something similar to that and would have conquered the world.Batman would be shown here fighting against his army by sneaking into some sort of lair of theirs in the desert but gets captured.
The following images could prove it more that darkseid has already conqured the Earth.
Well i think that the flash cameo would appear here almost at the end of the movie where he would run back into time where he would warn the others of that timeline of darkseid coming and thus maybe thats why zack synder used the hastag #unitetheseven when he released the pic of aquaman.
This would all happen at the ending of the movie as i think that it may be divided into 3 parts Batman vs superman Batman and superman and wonderwoman vs doomsday and then darkseid however it doesnt need to happen in that exact sequence the reason i think something like this would happen is because that dc wouldnt keep a regular film they would fill it with twists and turn and thats what i love them for. Batman vs Superman comes out on March 23rd 2016.