Arnold Wesker is quiet, shy, meek, weak, and every other adjective that seems un-villain-ly. But he develops his own version of Tyler Durden in the form of Scarface, a ventriloquist's dummy, built to look like an Edward G. Robinson-type old school gangster with a big fat cigar and a tommy gun (which works). Wesker believes that Scarface is real, an illusion more difficult for others to buy into because Wesker isn't a skilled ventriloquist and can't pronounce B's without moving his lips (forcing him to curse "Gatman"). People usually try to address Wesker, knowing him to be the man in control of the dummy, but Scarface belittles Wesker and demands to be respected. And he earns respect. Scarface somehow turns out to be a skilled criminal, and he is able to put together a real gang.
At various times in the comics, they play around with the idea that the doll is real and not just an aspect of Wesker's fractured mind. Recently, when Wesker is murdered, a woman finds the dummy and picks up right where Wesker left off, becoming the new Ventriloquist, but Scarface is almost exactly the same.
Pros: Scarface is a mob guy, and the first two movies have emphasized the mob. And with the Joker's line "You've changed things forever, there's no going back," they've signaled that the craziness will only escalate. So a lunatic barking out orders through a dummy almost makes sense.
Cons: They could have a ton of fun with a shadowy figure running a mob with a shocking reveal later in the film that it was a ventriloquist the whole time, but naming The Ventriloquist as the villain will be big enough news that everyone will know what's coming. Which is a shame.
A much bigger con is that the concept of the Ventriloquist is kind of silly. I actually like the character and I think he could work very well, but convincing Warner Bros. to spend $200 million to make a movie where the bad guy speaks through a dummy... well... that would be difficult.
How I'd do it: It would be a fairly straightforward story, except for the dummy part. Arnold Wesker is a loser in life, weak and defeated. He sees the dummy, maybe not even with the gangster look, maybe just an old ventriloquist's dummy with the big eyes. He starts talking to it, and it's angry. He figures out some way for it to fire a gun, and starts out with petty crime, making his way up. His associates laugh at him at first, but his cunning and his ruthlessness earn him respect and soon he's a threat to Gotham that Batman has to deal with.
Casting Guesses: Wesker is usually portrayed as an old guy but he doesn't necessarily have to be, as long as he seems perfectly harmless, but with a hidden mean streak. George Dzundza voiced both the Ventriloquist and Scarface on the animated series in the 90s and did an excellent job. Paul Giamatti, currently leading the Penguin poll, could be Wesker as well. But William H. Macy is a brilliant actor, can appear to be the kind of wimp that Wesker has to be, and have the hidden scariness of Scarface behind his eyes, so he would be my choice.
Verdict: It's a character that could really fit in the movie universe, since it combines the insanity of Gotham with the mafia that the movies have focused on, but far too many people would be turned off by the idea of Batman fighting some guy with a doll. I won't say there's no chance, he could make a cameo, but there's almost no way he'll be the focus of the movie.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.