Ever since I heard the rumours, and looked into the character,
Black mask could be extremely interesting to see. My only reservation being the idea of a deformed villain takes a little of the limelight from two face, though if Sionis is the one who does it to Den, it's somehow pretty logical. And visually he could be intimidating and somewhat unique. He links to Wayne and Batman, and his freakiness offers a nice counterpart to Falcone's ordinariness.
as a side villain I would love to see
The Penguin in a small role, just because I think there is another to do the character than has previoulsy been seen - in BR he was a Dickensian freak, now he could be the thug posing as a businessman, smuggling weapons in umbrella crates.
Good as he was, i dont think we need to see Scarecrow again, his fate was unclear, but thinking about it, to have the charcter dragged off into the unknown (the fog) screaming (from the taser) is, depsite offering no finality, a fitting last glimpse of him
And as for Ra's returning, I am completely undecided. If the entire trilogy is about Batman starting up, then a final encounter with the mentor would be fitting. If batman has already begun, it would be a step backwards.
Intersting that Gordon mentions a lack of Crane's body but doen't mention Ra's- even the writers seem unsure whether or not he's a loose end.