4. No Harley Quinn
With The Joker dead, many have speculated that his equally psychotic lover Harley Quinn would emerge as the primary villain for the third game. Though that seems natural enough, and perhaps I'm just prejudiced by what I know of Harley Quinn from the source material, but I don't think she's at all a good fit for the main villain. And no, it's not because she's a woman. Harley works best when she's at the Joker's side, his mental minion who carries out his deeds and is basically a nice buffer between the player and the main antagonist. Psychotic though she is, she's not anywhere near as imposing as the countless other classic Batman villains you could include (who we'll get to in a minute), so unless her narrative is wrapped up early in the game - she could be a mid-game boss - then I'm not at all interested in seeing her return. By all means hint that she's planning something, but placing her as the main antagonist would be weak sauce.