I feel like this would've had to have been settled in th interest check first to see how much people were interested in it. Then, depending on the viewers and possible participants, then make the OOC. 

If I may note, the way that you setup the story makes it as if Darkseid is unbeatable. Furthermore, with 90% of our heroes AND villains dead and gone, including the powerhouses, who is [b]really[/b] going to stand a chance against him? Impossible odds may be DC's specialty, but there isn't really a clear cut way to be able to pull this off.

My Suggestion: Maybe our characters are just on the brink of the earth being conquered but they are sent back in time by Dr. Fate or something to try and prevent that future from occurring. Fate determines that only those of lesser notoriety in terms of cosmic crisis can be sent back successfully. That could maybe have more interest behind it.

I get that you may want the whole post apocalypse vibe, but there isn't really a specific story to follow, making it a sandbox that's a little too big.