David Cain is yet another assassin, who's best known as the father of Cassandra Cain, the current Batgirl (Lady Shiva, another assassin, is Cassandra's mother). A former member of Ra's al-Ghul's League of Assassins, he is one of many people who trained Bruce Wayne as he prepared to become Batman.
In the Bruce Wayne: Murderer?/Bruce Wayne: Fugitive arcs (spoilers to follow), David Cain shows his attention to detail and intense preparation by sneaking into Wayne Manor undetected and murdering Bruce Wayne's ex-girlfriend, framing Bruce in such a way that his only defense is that he was in the city as Batman.
Pros: Assassins work well in the movie setting. He's a good foe for Batman because he has the same kind of obsessive nature.
Cons: He's a more recent and lesser-known assassin. Based on the poll here, Deadshot's probably a heck of a lot more popular. A lot of his story is tied to Cassandra, who we won't be seeing in a movie anytime soon (Batgirl is unlikely, but if we do get one you can be sure it'll be Barbara Gordon).
How I'd do it: Like all the other assassins, one of the criminal types Batman's ticked off in the first two movies hires Cain to take out the Bat.
Casting Guesses: He's an older guy, but still in good shape, and tough as nails. Terrence Stamp in The Limey is about right, but that's almost 10 years old now, and Terrence Stamp probably can't pull off the in shape/tough part as well anymore. I bet Eric Roberts could do it, but he was already in The Dark Knight. The best I can think of is Viggo Mortensen, who if you can believe it, will turn 50 later this year.
Verdict: Possible, but pretty unlikely relative to other assasins. We're more likely to see Cain than Zeiss, but Deadshot's a much better bet.
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.