There are still more characters to be revealed in Batman: Arkham City according to Dax Ginn, Marketing Manager at Rocksteady.

Speaking to Play Magazine he said;

Were pretty strict at Rocksteady in terms of what assets we allow
out of the studio, and everyone knows how important it is to make those
reveals have as much impact as possible weve still got some more
reveals to come.

We reinterpret those characters you can see our
reinterpretation of The Riddler is pretty dark, our interpretation of
Penguin is really dark so [DC/Warner] give us a lot of
creative freedom to push those characters in really interesting
directions. But, much like Arkham Asylum, we really want to make sure
theres a load of obscure nods to other aspects of Batmans history as
well, because Batman fans really dig that stuff.

So who could these new characters be?