Well, there are too many things that can go wrong with the next movie, but the writers they have onboard right now are very talented, so I don't think they will screw it up. Personally, I think that they might have Gordon's kid become Robin. They need to adapt the old characters to the new dark setting, and somehow make it seem more exciting than the last movie (which is going to be very hard).
I definitely see Johnny Depp as the Riddler, and you know what? If Philip Seymour Hoffman does the Penguin, then I have no doubt it will be cool. I just hope the writers supply him with a good script...
But think about this: If the third one is just as awesome as the Dark Knight, where could they possibly go with it? Poison Ivy and Mr. Freeze? I'm not feeling the love.
EDIT: It would actually be kinda cool if they incorporated Batzarro, but only if they did it right.
And instead of Robin, what about Nightwing? That would be kinda lame if they skipped Robin and went right to Nightwing, though.
Clicky clicky
^ Has to be the funniest Batman spoof ever.