My only worry for future films is that they start doubling up the villains. If they'd matched Harley Quinn up with the Joke that wouldn't have been bad, I've always thought of her as more of a sidekick (don't follow the comics so don't know if that holds true there).
Otherwise I hope they stick with the single villains. Penguin would be good, Two Face now that they've apparently, introduced him, Poison Ivy, Man-Bat, Mr. Freeze. How about Killer Croc? Since Spider-Man seemed to not want to get into the Lizard even after introducing Curt Connors, why not?
E...