I think the Ventriloquist is an interesting villain, but I don't think Nolan is going to include him because he's ANOTHER character with a split personality disorder, much like Two-Face, and Two-Face is already going to be involved in both The Dark Knight and the third movie.
Sure, A LOT of Batman villains deal with duality, but the other problem with the Ventriloquist is tying the villain to the hero, Batman, in a personal way. As we know, the best villains are those that tie into some aspect of the hero's personality or personal life or pyschology. I don't see the Arnold Wesker/the Ventriloquist doing this really.
Greg