With the return of the Multiverse to DC Comics, I'm most interested to see what the new Earth-3 looks like. I've always liked my evil opposites, so the Crime Syndicate and their whole reversed-morality world was always a favorite among DC's many parallel worlds. Now, according to this Newsarama article, the awkwardly-titled Countdown Presents The Search for Ray Palmer: Crime Syndicate #1 will give us our first look at the world where heroes are villains and evil always wins.
It looks like we're going to see that world expanded as well, with the introduction of the Jokester, presumably a heroic foil to the villainous Owlman, as well as a few other familiar faces. Sean McKeever will be writing the special, and mentions that in addition to introducing the Jokester, the story will solve a long-running DC Universe mystery. I'm going to speculate that the mystery is that of the parentage of long-running Teen Titans ally Duela Dent a.k.a. the Jokers Daughter, and that she's actually been the Jokester's daughter all along, hence her recent death at the hands of a monitor in Countdown.