After the Joker kills Nightwing, Batman flips and in rage, murders him. Then is never seen again, but presumed dead. (The character born in mystery, he should end in one.)
Wonder Woman after learns everything, she couldn't learn on paradise island falls in love with a man, gives up his powers and amazon persona so they can be a family. (it fits her so much better than Superman) Is killed by natural causes
Superman is almost killed by Lex Luthor, but Lex, in the last moment has mercy on him, cuz he realizes, with all the other heroes gone, Superman is a last line of defense, and a necessarry evil. (And now, he knows he can be stopped, since he did it.) With not being that consered with Superman anymore, Lex himself starts to protect humanity with whatever reasources he's got. When Superman is killed in an alien invasion, Lex has to finish the good fight alone.
With his enemies gone, Lex can't understand why he left, why he survived, and the heroes didn't? Can't find his place. Maybe the gods, the heroes were here to teach man kind a lesson. To show the way.
And, (since he always represented the best and the worst things about humanity), Lex, takes up the never ending battle alone. One day soon, there will be others to join him - there always are. But for now, he's alone, the worlds only Superhero and still not giving up - he learned his lesson the hard way. And his gonna teach a lot of people a lesson, before someone else takes his place too.
