We know that Harley, Joker and Deadshot are going to be part of the team. I'm just hoping that Deathstroke, Poison Ivy, Lex Luthor, Bronze Tiger and Bane make the cut. Also, the villains should be mostly Batman and Green Arrow villains since Batman has already been around for a while in this continuity and Green Arrow won't be showing up for a while.
I'd save the Flash, Aquaman and Superman enemies for their own films. Wonder Woman's foes are a bit to powerful to reign in and Green Lantern's are too cosmic. And yes, this means no Black manta or King Shark until Aquaman hits, no Parasite, Conduit or Metallo until we get a Man of Steel Sequel and no Captain Cold or Mirror Master until there's a Flash movie. And if any Wonder Woman enemy shows up, I'd like it to be Cheetah or Maxwell Lord since they don't have the same power level as somebody like Ares, Circe or even Giganta.