Batman: Dies in the line of duty.
Superman: Earth becomes a utopia and Superman spends the rest of his life inspiring new heroes across the Universe.
Hal Jordan: Becomes permanently one with the Central Power Battery and serves as a counter balance to some of the bad ideas the Guardians come up with.
Wonder Woman: Becomes full time ruler of the Amazons and a full Goddess of the Greek Pantheon.
Aquaman: Retires from superheroics and splits his time between ruling Atlantis and mentoring a new Aquaman
Martian Manhunter: Sacrifices himself to jump start a new Martian race
Cyborg: Becomes one with the world wide web and mentors a new generation of tech heroes
Booster Gold: Goes back to his future to serve out his sentence for stealing all of that stuff from the museum. Becomes a well known politician afterwards and makes his world a better place
Luthor: Dies impoverished and alone. Only Superman attends his funeral.
Joker: Likely dies in a final battle with Batman. Batman burns his remains.
John Stewart: Spends the rest of his life fighting for peace on a New Mosaic type world and trying to unite the many cultures living there.
Guy Gardner: Dies in the line of duty.
Kyle Raynor: Retires and spends the rest of his life using his ring to create great works of art that inspire people all over the Universe.
Barry Allen: Seems like he has to become one with the Speed Force I guess.
Wally West: See Barry Allen....
Dick Grayson: Retires after Batman's death. Realizes he doesn't need to become the new Batman with the army of sidekicks Batman has put together over the years. He becomes heavily involved in helping victims of crimes and realizes he can make a bigger difference this way.
Captain Atom: Sacrifices himself to become a free source of energy on Earth for 1000s of years.
Batgirl: After her father's death becomes a cop to honor his memory, eventually she becomes commissioner of Gotham.
Jason Todd: Becomes more unstable and violent as time goes by and becomes a full on villain. Killed by Damien Wayne who has replaced Bruce as Batman after his death.
Tim Drake: Retires after Batman's death, probably gets married and has kids. He becomes an Oracle like figure part time to help out new Bat family heroes.
Damien Wayne: Becomes Batman for a time after Bruce's death but eventually goes bad and becomes the successor to Ra's Al Ghul.