The Riddler...*Sigh* He's so lame.

If they're smart it'll be Catwoman, Mr. Freeze thanks to the fact that both have huge story lines. Another good possibility is a come back for The Joker (Yeah, I know, no one can replace Heath but don't be surprised if they find someone to try and pull it off.) and his girlfriend Harley Quinn!
Outside of those guys I can't really see any other villains entering in as nicely as the ones I've stated above thanks to how dark and deep the story line has been so far.
As for Christian's last Batman...I hope not, I mean my friends are afraid that the next Batman movies are going to suck, and the older ones all had different Batmans and for me that would be a bad sign. On another bad note they implied a "need" for Robin....And frankly with where the story has been going going that could be damn near impossible to add him in...Or to add in Batgirl for that matter. (******** sidekicks...) But I know they'll try if there is another, I mean.
But above all I hope they pull off something good for the next one(s) after, if there is going to be more.

WEE!!