Gotta agree with Hurly on that one. If Nolan doesn't want to do it, they can always find someone else to carry on in a similar fashion.
Batman should fight ninja robot pirates in the next film. Whatever villan they come up will pale in comparison to the two in this film. Black Mask isn't as well known to the casual Batman fan, Deadshot/Killer Croc were dealt with in Gotham Knights (somewhat?) easily, so there's little chance of those two making an appearance. Nolan has tried to make this semi-realistic, so don't expect Mr.Freeze-somesuperpowered dude. Bane even seems to be somewhat of a stretch.
The Riddler seems like the obvious choice.